chapter 7. safety of flight - amazon s3 · meteorology 7−1−1 chapter 7. safety of flight...

AIM8/15/19

7−1−1Meteorology

Chapter 7. Safety of Flight

Section 1. Meteorology

7−1−1. National Weather Service AviationWeather Service Program

a. Weather service to aviation is a joint effort of theNational Oceanic and Atmospheric Administration(NOAA), the National Weather Service (NWS), theFederal Aviation Administration (FAA), Departmentof Defense, and various private sector aviationweather service providers. Requirements for allaviation weather products originate from the FAA,which is the Meteorological Authority for the U.S.

b. NWS meteorologists are assigned to all airroute traffic control centers (ARTCC) as part of theCenter Weather Service Units (CWSU) as well as theAir Traffic Control System Command Center(ATCSCC). These meteorologists provide special-ized briefings as well as tailored forecasts to supportthe needs of the FAA and other users of the NAS.

c. Aviation Products

1. The NWS maintains an extensive surface,upper air, and radar weather observing program; anda nationwide aviation weather forecasting service.

2. Airport observations (METAR and SPECI)supported by the NWS are provided by automatedobserving systems.

3. Terminal Aerodrome Forecasts (TAF) areprepared by 123 NWS Weather Forecast Offices(WFOs) for over 700 airports. These forecasts arevalid for 24 or 30 hours and amended as required.

4. Inflight aviation advisories (for example,Significant Meteorological Information (SIGMETs)and Airmen’s Meteorological Information (AIR-METs)) are issued by three NWS MeteorologicalWatch Offices; the Aviation Weather Center (AWC)in Kansas City, MO, the Alaska Aviation WeatherUnit (AAWU) in Anchorage, AK, and the WFO inHonolulu, HI. Both the AWC and the AAWU issuearea forecasts (FA) for selected areas. In addition,NWS meteorologists assigned to most ARTCCs aspart of the Center Weather Service Unit (CWSU)provide Center Weather Advisories (CWAs) and

gather weather information to support the needs ofthe FAA and other users of the system.

5. Several NWS National Centers for Environ-mental Production (NCEP) provide aviation specificweather forecasts, or select public forecasts which areof interest to pilots and operators.

(a) The Aviation Weather Center (AWC)displays a variety of domestic and internationalaviation forecast products over the Internet ataviationweather.gov.

(b) The NCEP Central Operations (NCO) isresponsible for the operation of many numericalweather prediction models, including those whichproduce the many wind and temperature aloftforecasts.

(c) The Storm Prediction Center (SPC) issuestornado and severe weather watches along with otherguidance forecasts.

(d) The National Hurricane Center (NHC)issues forecasts on tropical weather systems (forexample, hurricanes).

(e) The Space Weather Prediction Center(SWPC) provides alerts, watches, warnings andforecasts for space weather events (for example, solarstorms) affecting or expected to affect Earth’senvironment.

(f) The Weather Prediction Center (WPC)provides analysis and forecast products on a nationalscale including surface pressure and frontal analyses.

6. NOAA operates two Volcanic Ash AdvisoryCenters (VAAC) which issue forecasts of ash cloudsfollowing a volcanic eruption in their area ofresponsibility.

7. Details on the products provided by the abovelisted offices and centers is available in FAAAdvisory Circular 00-45, Aviation Weather Services.

d. Weather element values may be expressed byusing different measurement systems depending onseveral factors, such as whether the weather productswill be used by the general public, aviation interests,international services, or a combination of these

AIM 8/15/19

7−1−2 Meteorology

users. FIG 7−1−1 provides conversion tables for themost used weather elements that will be encounteredby pilots.

7−1−2. FAA Weather Services

a. The FAA provides the Flight Service program,which serves the weather needs of pilots through itsflight service stations (FSS) (both government andcontract via 1-800-WX-BRIEF) and via the Internet,through Leidos Flight Service.

b. The FAA maintains an extensive surfaceweather observing program. Airport observations(METAR and SPECI) in the U.S. are provided byautomated observing systems. Various levels ofhuman oversight of the METAR and SPECI reportsand augmentation may be provided at select largerairports by either government or contract personnelqualified to report specified weather elements thatcannot be detected by the automated observingsystem.

c. Other Sources of Weather Information

1. In Alaska, Telephone Information BriefingService (TIBS) (FSS); Transcribed Weather Broad-cast (TWEB) locations, and telephone access to theTWEB (TEL−TWEB) provide continuously updatedrecorded weather information for short or localflights. Separate paragraphs in this section giveadditional information regarding these services.

REFERENCE−AIM, Paragraph 7−1−8 , Telephone Information Briefing Service (TIBS)(Alaska only)AIM, Paragraph 7−1−9 , Transcribed Weather Broadcast (TWEB)(Alaska Only)

2. Weather and aeronautical information arealso available from numerous private industrysources on an individual or contract pay basis.Information on how to obtain this service should beavailable from local pilot organizations.

3. Pilots can access Leidos Flight Services viathe Internet. Pilots can receive preflight weather dataand file domestic VFR and IFR flight plans. Thefollowing is the FAA contract vendor:

Leidos Flight Service

Internet Access: http://www.1800wxbrief.comFor customer service: 1−800−WXBRIEF

7−1−3. Use of Aviation Weather Products

a. Air carriers and operators certificated under theprovisions of 14 CFR Part 119 are required to use theaeronautical weather information systems defined inthe Operations Specifications issued to that certifi-cate holder by the FAA. These systems may utilizebasic FAA/National Weather Service (NWS) weatherservices, contractor− or operator−proprietary weath-er services and/or Enhanced Weather InformationSystem (EWINS) when approved in the OperationsSpecifications. As an integral part of this systemapproval, the procedures for collecting, producingand disseminating aeronautical weather information,as well as the crew member and dispatcher training tosupport the use of system weather products, must beaccepted or approved.

b. Operators not certificated under the provisionsof 14 CFR Part 119 are encouraged to use FAA/NWSproducts through Flight Service Stations, LeidosFlight Service, and/or Flight Information Services−Broadcast (FIS−B).

c. The suite of available aviation weather producttypes is expanding, with the development of newsensor systems, algorithms and forecast models. TheFAA and NWS, supported by various weatherresearch laboratories and corporations under contractto the Government, develop and implement newaviation weather product types. The FAA’s NextGenAviation Weather Research Program (AWRP)facilitates collaboration between the NWS, the FAA,and various industry and research representatives.This collaboration ensures that user needs andtechnical readiness requirements are met beforeexperimental products mature to operational applica-tion.

d. The AWRP manages the transfer of aviationweather R&D to operational use through technicalreview panels and conducting safety assessments toensure that newly developed aviation weatherproducts meet regulatory requirements and enhancesafety.

AIM8/15/19


FIG 7−1−1

Weather Elements Conversion Tables

AIM 8/15/19


e. The AWRP review and decision−makingprocess applies criteria to weather products at variousstages . The stages are composed of the following:

1. Sponsorship of user needs.

2. R & D and controlled testing.

3. Experimental application.

4. Operational application.

f. Pilots and operators should be aware thatweather services provided by entities other than FAA,NWS, or their contractors may not meet FAA/NWSquality control standards. Hence, operators and pilotscontemplating using such services should requestand/or review an appropriate description of servicesand provider disclosure. This should include, but isnot limited to, the type of weather product (forexample, current weather or forecast weather), thecurrency of the product (that is, product issue andvalid times), and the relevance of the product. Pilotsand operators should be cautious when usingunfamiliar products, or products not supported byFAA/NWS technical specifications.

NOTE−When in doubt, consult with a FAA Flight Service StationSpecialist.

g. In addition, pilots and operators should beaware there are weather services and productsavailable from government organizations beyond thescope of the AWRP process mentioned earlier in thissection. For example, governmental agencies such asthe NWS and the Aviation Weather Center (AWC), orresearch organizations such as the National Centerfor Atmospheric Research (NCAR) display weather“model data” and “experimental” products whichrequire training and/or expertise to properly interpretand use. These products are developmental proto-types that are subject to ongoing research and canchange without notice. Therefore, some data ondisplay by government organizations, or governmentdata on display by independent organizations may beunsuitable for flight planning purposes. Operatorsand pilots contemplating using such services shouldrequest and/or review an appropriate description ofservices and provider disclosure. This should include,but is not limited to, the type of weather product (forexample, current weather or forecast weather), thecurrency of the product (i.e., product issue and validtimes), and the relevance of the product. Pilots and

operators should be cautious when using unfamiliarweather products.NOTE−When in doubt, consult with a FAA Flight Service StationSpecialist.

h. With increased access to weather products viathe public Internet, the aviation community hasaccess to an over whelming amount of weatherinformation and data that support self-briefing. FAAAC 00-45 (current edition) describes the weatherproducts distributed by the NWS. Pilots andoperators using the public Internet to access weatherfrom a third party vendor should request and/orreview an appropriate description of services andprovider disclosure. This should include, but is notlimited to, the type of weather product (for example,current weather or forecast weather), the currency ofthe product (i.e., product issue and valid times), andthe relevance of the product. Pilots and operatorsshould be cautious when using unfamiliar weatherproducts and when in doubt, consult with a FlightService Specialist.

i. The development of new weather products,coupled with the termination of some legacy textualand graphical products may create confusion betweenregulatory requirements and the new products. Allflight−related, aviation weather decisions must bebased on all available pertinent weather products. Asevery flight is unique and the weather conditions forthat flight vary hour by hour, day to day, multipleweather products may be necessary to meet aviationweather regulatory requirements. Many new weatherproducts now have a Precautionary Use Statementthat details the proper use or application of thespecific product.

j. The FAA has identified three distinct types ofweather information available to pilots and operators.

1. Observations. Raw weather data collectedby some type of sensor suite including surface andairborne observations, radar, lightning, satelliteimagery, and profilers.

2. Analysis. Enhanced depiction and/or inter-pretation of observed weather data.

3. Forecasts. Predictions of the developmentand/or movement of weather phenomena based onmeteorological observations and various mathemati-cal models.

k. Not all sources of aviation weather informationare able to provide all three types of weather

AIM8/15/19


information. The FAA has determined that operatorsand pilots may utilize the following approved sourcesof aviation weather information:

1. Federal Government. The FAA and NWScollect raw weather data, analyze the observations,and produce forecasts. The FAA and NWSdisseminate meteorological observations, analyses,and forecasts through a variety of systems. Inaddition, the Federal Government is the onlyapproval authority for sources of weather observa-tions; for example, contract towers and airportoperators may be approved by the FederalGovernment to provide weather observations.

2. Enhanced Weather Information System(EWINS). An EWINS is an FAA authorized,proprietary system for tracking, evaluating, report-ing, and forecasting the presence or lack of adverseweather phenomena. The FAA authorizes a certifi-cate holder to use an EWINS to produce flightmovement forecasts, adverse weather phenomenaforecasts, and other meteorological advisories. Formore detailed information regarding EWINS, see theAviation Weather Services Advisory Circular 00−45and the Flight Standards Information ManagementSystem 8900.1.

3. Commercial Weather InformationProviders. In general, commercial providers pro-duce proprietary weather products based onNWS/FAA products with formatting and layoutmodifications but no material changes to the weatherinformation itself. This is also referred to as“repackaging.” In addition, commercial providersmay produce analyses, forecasts, and other propri-etary weather products that substantially alter theinformation contained in government−producedproducts. However, those proprietary weatherproducts that substantially alter government−pro-duced weather products or information, may only beapproved for use by 14 CFR Part 121 and Part 135certificate holders if the commercial provider isEWINS qualified.

NOTE−Commercial weather information providers contracted byFAA to provide weather observations, analyses, andforecasts (e.g., contract towers) are included in the FederalGovernment category of approved sources by virtue ofmaintaining required technical and quality assurancestandards under Federal Government oversight.

7−1−4. Graphical Forecasts for Aviation(GFA)

a. The GFA website is intended to provide thenecessary aviation weather information to give usersa complete picture of the weather that may affectflight in the continental United States (CONUS). Thewebsite includes observational data, forecasts, andwarnings that can be viewed from 14 hours in the pastto 15 hours in the future, including thunderstorms,clouds, flight category, precipitation, icing, turbu-lence, and wind. Hourly model data and forecasts,including information on clouds, flight category,precipitation, icing, turbulence, wind, and graphicaloutput from the National Weather Service’s (NWS)National Digital Forecast Data (NDFD) are available.Wind, icing, and turbulence forecasts are available in3,000 ft increments from the surface up to 30,000 ftMSL, and in 6,000 ft increments from 30,000 ft MSLto 48,000 ft MSL. Turbulence forecasts are alsobroken into low (below 18,000 ft MSL) and high (ator above 18,000 ft MSL) graphics. A maximum icinggraphic and maximum wind velocity graphic(regardless of altitude) are also available. Built withmodern geospatial information tools, users can panand zoom to focus on areas of greatest interest. Targetusers are commercial and general aviation pilots,operators, briefers, and dispatchers.

b. Weather Products.

1. The Aviation Forecasts include griddeddisplays of various weather parameters as well asNWS textual weather observations, forecasts, andwarnings. Icing, turbulence, and wind griddedproducts are three−dimensional. Other griddedproducts are two−dimensional and may represent a“composite” of a three−dimensional weather phe-nomenon or a surface weather variable, such ashorizontal visibility. The following are examples ofaviation forecasts depicted on the GFA:

(a) Terminal Aerodrome Forecast (TAF)

(b) Ceiling & Visibility (CIG/VIS)

(c) Clouds

(d) Precipitation / Weather (PCPN/WX)

(e) Thunderstorm (TS)

(f) Winds

(g) Turbulence

(h) Ice

AIM 8/15/19


2. Observations & Warnings (Obs/Warn).The Obs/Warn option provides an option to displayweather data for the current time and the previous14 hours (rounded to the nearest hour). Users mayadvance through time using the arrow buttons or byclicking on the desired hour. Provided below are theObs/Warn product tabs available on the GFA website:

(a) METAR

(b) Precipitation/Weather (PCPN/WX)

(c) Ceiling & Visibility (CIG/VIS)

(d) Pilot Reports (PIREP)

(e) Radar & Satellite (RAD/SAT)

3. The GFA will be continuously updated andavailable online at http://new.aviationweather.gov/areafcst. Upon clicking the link above, select INFOon the top right corner of the map display. The nextscreen presents the option of selecting Overview,Products, and Tutorial. Simply select the tab ofinterest to explore the enhanced digital and graphicalweather products designed to replace the legacy FA.Users should also refer to AC 00−45, Aviation

Weather Services, for more detailed information onthe GFA.

4. GFA Static Images. Some users withlimited internet connectivity may access staticimages via the Aviation Weather Center (AWC) at:http://www.aviationweather.gov/gfa/plot. Thereare two static graphical images available, titledAviation Cloud Forecast and Aviation SurfaceForecast. The Aviation Cloud Forecast providescloud coverage, bases, layers, and tops with AirmetSierra for mountain obscuration and Airmet Zulu foricing overlaid. The Aviation Surface Forecastprovides visibility, weather phenomena, and winds(including wind gusts) with Airmet Sierra forinstrument flight rules conditions and Airmet Tangofor sustained surface winds of 30 knots or moreoverlaid. These images are presented on ten separatemaps providing forecast views for the entire CONUSon one and nine regional views which provide moredetail for the user. They are updated every 3 hours andprovide forecast snapshots for 3, 6, 9, 12, 15, and18 hours into the future. (See FIG 7−1−2 andFIG 7−1−3.)

FIG 7−1−2

Aviation Surface Forecast

AIM8/15/19


FIG 7−1−3

Aviation Cloud Forecast

7−1−5. Preflight Briefing

a. Flight Service Stations (FSS) are the primarysources for obtaining preflight briefings and to fileflight plans by phone or the Internet. Flight ServiceSpecialists are qualified and certified as PilotWeather Briefers by the FAA. They are not authorizedto make original forecasts, but are authorized totranslate and interpret available forecasts and reportsdirectly into terms describing the weather conditionswhich can be expected along the flight route and at thedestination. Three basic types of preflight briefings(Standard, Abbreviated, and Outlook) are availableto serve the pilot’s specific needs. Pilots shouldspecify to the briefer the type of briefing they want,along with their appropriate background information.This will enable the briefer to tailor the informationto the pilot’s intended flight. The followingparagraphs describe the types of briefings availableand the information provided in each briefing.

REFERENCE−AIM, Paragraph 5−1−1 , Preflight Preparation, for items that arerequired.

b. Standard Briefing. You should request aStandard Briefing any time you are planning a flightand you have not received a previous briefing or havenot received preliminary information through massdissemination media; for example, in Alaska only,TIBS and TWEB. International data may be

inaccurate or incomplete. If you are planning a flightoutside of U.S. controlled airspace, the briefer willadvise you to check data as soon as practical afterentering foreign airspace, unless you advise that youhave the international cautionary advisory. Thebriefer will automatically provide the followinginformation in the sequence listed, except as noted,when it is applicable to your proposed flight.

1. Adverse Conditions. Significant meteoro-logical and/or aeronautical information that mightinfluence the pilot to alter or cancel the proposedflight; for example, hazardous weather conditions,airport closures, air traffic delays, etc. Pilots shouldbe especially alert for current or forecast weatherthat could reduce flight minimums below VFR orIFR conditions. Pilots should also be alert for anyreported or forecast icing if the aircraft is not certifiedfor operating in icing conditions. Flying into areasof icing or weather below minimums could havedisastrous results.

2. VFR Flight Not Recommended. WhenVFR flight is proposed and sky conditions orvisibilities are present or forecast, surface or aloft,that, in the briefer’s judgment, would make flightunder VFR doubtful, the briefer will describe theconditions, describe the affected locations, and usethe phrase “VFR flight not recommended.” This

AIM 8/15/19


recommendation is advisory in nature. The finaldecision as to whether the flight can be conductedsafely rests solely with the pilot. Upon receiving a“VFR flight not recommended” statement, thenon−IFR rated pilot will need to make a “go or no go”decision. This decision should be based on weighingthe current and forecast weather conditions againstthe pilot’s experience and ratings. The aircraft’sequipment, capabilities and limitations should alsobe considered.

NOTE−Pilots flying into areas of minimal VFR weather couldencounter unforecasted lowering conditions that place theaircraft outside the pilot’s ratings and experience level.This could result in spatial disorientation and/or loss ofcontrol of the aircraft.

3. Synopsis. A brief statement describing thetype, location and movement of weather systemsand/or air masses which might affect the proposedflight.

NOTE−These first 3 elements of a briefing may be combined in anyorder when the briefer believes it will help to more clearlydescribe conditions.

4. Current Conditions. Reported weatherconditions applicable to the flight will be summarizedfrom all available sources; e.g., METARs/ SPECIs,PIREPs, RAREPs. This element will be omitted if theproposed time of departure is beyond 2 hours, unlessthe information is specifically requested by the pilot.

5. En Route Forecast. Forecast en routeconditions for the proposed route are summarized inlogical order; i.e., departure/climbout, en route, anddescent. (Heights are MSL, unless the contractions“AGL” or “CIG” are denoted indicating that heightsare above ground.)

6. Destination Forecast. The destination fore-cast for the planned ETA. Any significant changeswithin 1 hour before and after the planned arrival areincluded.

7. Winds Aloft. Forecast winds aloft will beprovided using degrees of the compass. The brieferwill interpolate wind directions and speeds betweenlevels and stations as necessary to provide expectedconditions at planned altitudes. (Heights are MSL.)Temperature information will be provided on request.

8. Notices to Airmen (NOTAMs).

(a) Available NOTAM (D) information perti-nent to the proposed flight, including special useairspace (SUA) NOTAMs for restricted areas, aerialrefueling, and night vision goggles (NVG).

NOTE−Other SUA NOTAMs (D), such as military operationsarea (MOA), military training route (MTR), and warningarea NOTAMs, are considered “upon request” briefingitems as indicated in paragraph 7−1−4b10(a).

(b) Prohibited Areas P−40, P−49, P−56,and the special flight rules area (SFRA) forWashington, DC.

(c) FSS briefers do not provide FDC NOTAMinformation for special instrument approach proce-dures unless specifically asked. Pilots authorized bythe FAA to use special instrument approachprocedures must specifically request FDC NOTAMinformation for these procedures.

NOTE−1. NOTAM information may be combined with currentconditions when the briefer believes it is logical to do so.

2. Airway NOTAMs, procedural NOTAMs, and NOTAMsthat are general in nature and not tied to a specificairport/facility (for example, flight advisories andrestrictions, open duration special security instructions,and special flight rules areas) are briefed solely by pilotrequest. NOTAMs, graphic notices, and other informationpublished in the NTAP are not included in pilot briefingsunless the pilot specifically requests a review of thispublication. For complete flight information, pilots areurged to review the printed information in the NTAP andthe Chart Supplement U.S. In addition to obtaining abriefing.

9. ATC Delays. Any known ATC delays andflow control advisories which might affect theproposed flight.

10. Pilots may obtain the following fromflight service station briefers upon request:

(a) Information on SUA and SUA−relatedairspace, except those listed in paragraph 7−1−4b8.

NOTE−1. For the purpose of this paragraph, SUA and relatedairspace includes the following types of airspace: alertarea, military operations area (MOA), warning area, andair traffic control assigned airspace (ATCAA). MTR dataincludes the following types of airspace: IFR trainingroutes (IR), VFR training routes (VR), and slow trainingroutes (SR).

2. Pilots are encouraged to request updated informationfrom ATC facilities while in flight.

AIM8/15/19


(b) A review of airway NOTAMs, proceduralNOTAMs, and NOTAMs that are general in natureand not tied to a specific airport/facility (for example,flight advisories and restrictions, open durationspecial security instructions, and special flight rulesareas), graphic notices, and other informationpublished in the NTAP.

(c) Approximate density altitude data.

(d) Information regarding such items as airtraffic services and rules, customs/immigrationprocedures, ADIZ rules, search and rescue, etc.

(e) GPS RAIM availability for 1 hour beforeto 1 hour after ETA or a time specified by the pilot.

(f) Other assistance as required.

c. Abbreviated Briefing. Request an Abbreviat-ed Briefing when you need information tosupplement mass disseminated data, update aprevious briefing, or when you need only one or twospecific items. Provide the briefer with appropriatebackground information, the time you received theprevious information, and/or the specific itemsneeded. You should indicate the source of theinformation already received so that the briefer canlimit the briefing to the information that you have notreceived, and/or appreciable changes in meteorologi-cal/aeronautical conditions since your previousbriefing. To the extent possible, the briefer willprovide the information in the sequence shown for aStandard Briefing. If you request only one or twospecific items, the briefer will advise you if adverseconditions are present or forecast. (Adverse condi-tions contain both meteorological and/or aeronauticalinformation.) Details on these conditions will beprovided at your request. International data may beinaccurate or incomplete. If you are planning a flightoutside of U.S. controlled airspace, the briefer willadvise you to check data as soon as practical afterentering foreign airspace, unless you advise that youhave the international cautionary advisory.

d. Outlook Briefing. You should request anOutlook Briefing whenever your proposed time ofdeparture is six or more hours from the time of thebriefing. The briefer will provide available forecastdata applicable to the proposed flight. This type ofbriefing is provided for planning purposes only. Youshould obtain a Standard or Abbreviated Briefingprior to departure in order to obtain such items as

adverse conditions, current conditions, updatedforecasts, winds aloft and NOTAMs, etc.

e. When filing a flight plan only, you will be askedif you require the latest information on adverseconditions pertinent to the route of flight.

f. Inflight Briefing. You are encouraged toobtain your preflight briefing by telephone or inperson before departure. In those cases where youneed to obtain a preflight briefing or an update to aprevious briefing by radio, you should contact thenearest FSS to obtain this information. Aftercommunications have been established, advise thespecialist of the type briefing you require and provideappropriate background information. You will beprovided information as specified in the aboveparagraphs, depending on the type of briefingrequested. En Route advisories tailored to the phaseof flight that begins after climb-out and ends withdescent to land are provided upon pilot request. Pilotsare encouraged to provide a continuous exchange ofinformation on weather, winds, turbulence, flightvisibility, icing, etc., between pilots and inflightspecialists. Pilots should report good weather as wellas bad, and confirm expected conditions as well asunexpected. Remember that weather conditions canchange rapidly and that a “go or no go” decision, asmentioned in paragraph 7−1−4b2, should be assessedat all phases of flight.

g. Following any briefing, feel free to ask for anyinformation that you or the briefer may have missedor are not understood. This way, the briefer is able topresent the information in a logical sequence, andlessens the chance of important items beingoverlooked.

7−1−6. Inflight Aviation Weather Advisories

a. Background

1. Inflight Aviation Weather Advisories areforecasts to advise en route aircraft of development ofpotentially hazardous weather. Inflight aviationweather advisories in the conterminous U.S. areissued by the Aviation Weather Center (AWC) inKansas City, MO, as well as 20 Center WeatherService Units (CWSU) associated with ARTCCs.AWC also issues advisories for portions of the Gulfof Mexico, Atlantic and Pacific Oceans, which areunder the control of ARTCCs with Oceanic flightinformation regions (FIRs). The Weather ForecastOffice (WFO) in Honolulu issues advisories for the

AIM 8/15/19


Hawaiian Islands and a large portion of the PacificOcean. In Alaska, the Alaska Aviation Weather Unit(AAWU) issues inflight aviation weather advisoriesalong with the Anchorage CWSU. All heights arereferenced MSL, except in the case of ceilings (CIG)which indicate AGL.

2. There are four types of inflight aviationweather advisories: the SIGMET, the ConvectiveSIGMET, the AIRMET (text or graphical product),and the Center Weather Advisory (CWA). All of theseadvisories use the same location identifiers (eitherVORs, airports, or well−known geographic areas) todescribe the hazardous weather areas.

3. The Severe Weather Watch Bulletins (WWs),(with associated Alert Messages) (AWW) supple-ments these Inflight Aviation Weather Advisories.

b. SIGMET (WS)/AIRMET (WA orG−AIRMET)

SIGMETs/AIRMET text (WA) products are issuedcorresponding to the Area Forecast (FA) areasdescribed in FIG 7−1−4 and FIG 7−1−5. Themaximum forecast period is 4 hours for SIGMETsand 6 hours for AIRMETs. The G−AIRMET is issuedover the CONUS every 6 hours, valid at 3−hourincrements through 12 hours with optional forecastspossible during the first 6 hours. The first 6 hours ofthe G−AIRMET correspond to the 6−hour period ofthe AIRMET. SIGMETs and AIRMETs are consid-ered “widespread” because they must be eitheraffecting or be forecasted to affect an area of at least3,000 square miles at any one time. However, if thetotal area to be affected during the forecast period isvery large, it could be that in actuality only a smallportion of this total area would be affected at any onetime.

1. SIGMETs/AIRMET (or G−AIRMET) for theconterminous U.S. (CONUS)

SIGMETs/AIRMET text products for the CONUSare issued corresponding to the areas in FIG 7−1−4.The maximum forecast period for a CONUSSIGMET is 4 hours and 6 hours for CONUSAIRMETs. The G−AIRMET is issued over theCONUS every 6 hours, valid at 3−hour incrementsthrough 12 hours with optional forecasts possibleduring the first 6 hours. The first 6 hours of theG−AIRMET correspond to the 6−hour period of theAIRMET. SIGMETs and AIRMETs are considered“widespread” because they must be either affecting

or be forecasted to affect an area of at least 3,000square miles at any one time. However, if the totalarea to be affected during the forecast period is verylarge, it could be that in actuality only a small portionof this total area would be affected at any one time.Only SIGMETs for the CONUS are for non-convec-tive weather. The U.S. issues a special category ofSIGMETs for convective weather called ConvectiveSIGMETs.

2. SIGMETs/AIRMETs for Alaska

Alaska SIGMETs are valid for up to 4 hours, exceptfor Volcanic Ash Cloud SIGMETs which are valid forup to 6 hours. Alaska AIRMETs are valid for up to8 hours.

3. SIGMETs/AIRMETs for Hawaii and U.S.FIRs in the Gulf of Mexico, Caribbean, WesternAtlantic and Eastern and Central Pacific Oceans

These SIGMETs are valid for up to 4 hours, exceptSIGMETs for Tropical Cyclones and Volcanic AshClouds, which are valid for up to 6 hours. AIRMETsare issued for the Hawaiian Islands and are valid forup to 6 hours. No AIRMETs are issued for U.S. FIRsin the the Gulf of Mexico, Caribbean, WesternAtlantic and Pacific Oceans.

c. SIGMET

A SIGMET advises of weather that is potentiallyhazardous to all aircraft. SIGMETs are unscheduledproducts that are valid for 4 hours. However,SIGMETs associated with tropical cyclones andvolcanic ash clouds are valid for 6 hours.Unscheduled updates and corrections are issued asnecessary.

1. In the CONUS, SIGMETs are issued whenthe following phenomena occur or are expected tooccur:

(a) Severe icing not associated with thunder-storms.

(b) Severe or extreme turbulence or clear airturbulence (CAT) not associated with thunderstorms.

(c) Widespread dust storms or sandstormslowering surface visibilities to below 3 miles.

(d) Volcanic ash.

2. In Alaska and Hawaii, SIGMETs are alsoissued for:

(a) Tornadoes.

AIM8/15/19


(b) Lines of thunderstorms.

(c) Embedded thunderstorms.

(d) Hail greater than or equal to 3/4 inch indiameter.

3. SIGMETs are identified by an alphabeticdesignator from November through Yankee exclud-ing Sierra and Tango. (Sierra, Tango, and Zulu arereserved for AIRMET text [WA] products;G−AIRMETS do not use the Sierra, Tango, or Zuludesignators.) The first issuance of a SIGMET will belabeled as UWS (Urgent Weather SIGMET).Subsequent issuances are at the forecaster’s discre-tion. Issuance for the same phenomenon will besequentially numbered, using the original designatoruntil the phenomenon ends. For example, the firstissuance in the Chicago (CHI) FA area forphenomenon moving from the Salt Lake City (SLC)FA area will be SIGMET Papa 3, if the previous twoissuances, Papa 1 and Papa 2, had been in the SLC FAarea. Note that no two different phenomena across thecountry can have the same alphabetic designator atthe same time.

EXAMPLE−Example of a SIGMET:BOSR WS 050600SIGMET ROMEO 2 VALID UNTIL 051000ME NH VTFROM CAR TO YSJ TO CON TO MPV TO CAROCNL SEV TURB BLW 080 EXP DUE TO STG NWLYFLOW. CONDS CONTG BYD 1000Z.

d. Convective SIGMET (WST)

1. Convective SIGMETs are issued in theconterminous U.S. for any of the following:

(a) Severe thunderstorm due to:

(1) Surface winds greater than or equal to50 knots.

(2) Hail at the surface greater than or equalto 3/4 inches in diameter.

(3) Tornadoes.

(b) Embedded thunderstorms.

(c) A line of thunderstorms.

(d) Thunderstorms producing precipitationgreater than or equal to heavy precipitation affecting40 percent or more of an area at least 3,000 squaremiles.

2. Any convective SIGMET implies severe orgreater turbulence, severe icing, and low−level windshear. A convective SIGMET may be issued for anyconvective situation that the forecaster feels ishazardous to all categories of aircraft.

3. Convective SIGMET bulletins are issued forthe western (W), central (C), and eastern (E) UnitedStates. (Convective SIGMETs are not issued forAlaska or Hawaii.) The areas are separated at 87 and107 degrees west longitude with sufficient overlap tocover most cases when the phenomenon crosses theboundaries. Bulletins are issued hourly at H+55.Special bulletins are issued at any time as requiredand updated at H+55. If no criteria meetingconvective SIGMET requirements are observed orforecasted, the message “CONVECTIVE SIGMET...NONE” will be issued for each area at H+55.Individual convective SIGMETs for each area (W, C,E) are numbered sequentially from number one eachday, beginning at 00Z. A convective SIGMET for acontinuing phenomenon will be reissued every hourat H+55 with a new number. The text of the bulletinconsists of either an observation and a forecast or justa forecast. The forecast is valid for up to 2 hours.

EXAMPLE−CONVECTIVE SIGMET 44CVALID UNTIL 1455ZAR TX OKFROM 40NE ADM-40ESE MLC-10W TXK-50WNWLFK-40ENE SJT-40NE ADMAREA TS MOV FROM 26025KT. TOPS ABV FL450.OUTLOOK VALID 061455-061855FROM 60WSW OKC-MLC-40N TXK-40WSWIGB-VUZ-MGM-HRV-60S BTR-40NIAH-60SW SJT-40ENE LBB-60WSW OKCWST ISSUANCES EXPD. REFER TO MOST RECENTACUS01 KWNS FROM STORM PREDICTION CENTERFOR SYNOPSIS AND METEOROLOGICAL DETAILS

AIM 8/15/19


FIG 7−1−4

SIGMET and AIRMET Locations − Conterminous United States

FIG 7−1−5

Hawaii Area Forecast Locations

AIM8/15/19


e. SIGMET Outside the CONUS

1. Three NWS offices have been designated byICAO as Meteorological Watch Offices (MWOs).These offices are responsible for issuing SIGMETsfor designated areas outside the CONUS that includeAlaska, Hawaii, portions of the Atlantic and PacificOceans, and the Gulf of Mexico.

2. The offices which issue international SIG-METs are:

(a) The AWC in Kansas City, Missouri.

(b) The AAWU in Anchorage, Alaska.

(c) The WFO in Honolulu, Hawaii.

3. SIGMETs for outside the CONUS are issuedfor 6 hours for volcanic ash clouds, 6 hours fortropical cyclones (e.g. hurricanes and tropicalstorms), and 4 hours for all other events. Like theCONUS SIGMETs, SIGMETs for outside theCONUS are also identified by an alphabeticdesignator from Alpha through Mike and arenumbered sequentially until that weather phenome-non ends. The criteria for an international SIGMETare:

(a) Thunderstorms occurring in lines, embed-ded in clouds, or in large areas producing tornadoesor large hail.

(b) Tropical cyclones.

(c) Severe icing.

(d) Severe or extreme turbulence.

(e) Dust storms and sandstorms loweringvisibilities to less than 3 miles.

(f) Volcanic ash.

EXAMPLE−Example of SIGMET Outside the U.S.:WSNT06 KKCI 022014SIGA0FKZMA KZNY TJZS SIGMET FOXTROT 3 VALID022015/030015 KKCI− MIAMI OCEANIC FIR NEWYORK OCEANIC FIR SAN JUAN FIR FRQ TS WI AREABOUNDED BY 2711N6807W 2156N6654W 2220N7040W2602N7208W 2711N6807W. TOPS TO FL470. MOV NE15KT. WKN. BASED ON SAT AND LTG OBS.MOSHER

f. AIRMET

1. AIRMETs (WAs) are advisories of signifi-cant weather phenomena but describe conditions at

intensities lower than those which require theissuance of SIGMETs. AIRMETs are intended fordissemination to all pilots in the preflight and en routephase of flight to enhance safety. AIRMETinformation is available in two formats: text bulletins(WA) and graphics (G−AIRMET). Both formats meetthe criteria of paragraph 7−1−3i and are issued on ascheduled basis every 6 hours beginning at0245 UTC. Unscheduled updates and corrections areissued as necessary. AIRMETs contain details aboutIFR, extensive mountain obscuration, turbulence,strong surface winds, icing, and freezing levels.

2. There are three AIRMETs: Sierra, Tango,and Zulu. After the first issuance each day, scheduledor unscheduled bulletins are numbered sequentiallyfor easier identification.

(a) AIRMET Sierra describes IFR conditionsand/or extensive mountain obscurations.

(b) AIRMET Tango describes moderateturbulence, sustained surface winds of 30 knots orgreater, and/or nonconvective low−level wind shear.

(c) AIRMET Zulu describes moderate icingand provides freezing level heights.

EXAMPLE−Example of AIRMET Sierra issued for the Chicago FAarea:CHIS WA 131445AIRMET SIERRA UPDT 2 FOR IFR AND MTN OBSCNVALID UNTIL 132100.AIRMET IFR...KY FROM 20SSW HNN TO HMV TO 50ENE DYR TO20SSWHNN CIG BLW 010/VIS BLW 3SM PCPN/BR/FG. CONDSENDG BY 18Z. .AIRMET IFR....MN LSFROM INL TO 70W YQT TO 40ENE DLH TO 30WNW DLH TO 50SE GFK TO 20 ENE GFK TOINL CIG BLW 010/VIS BLW 3SM BR. CONDS ENDG 15− 18Z. .AIRMET IFR....KSFROM 30N SLN TO 60E ICT TO 40S ICT TO 50WLBL TO 30SSW GLD TO 30N SLNCIG BLW 010/VIS BLW 3SM PCPN/BR/FG. CONDSENDG 15−18Z..AIRMET MTN OBSCN...KY TNFROM HNN TO HMV TO GQO TO LOZ TO HNNMTN OBSC BY CLDS/PCPN/BR. CONDS CONTG

AIM 8/15/19


BYD 21Z THRU 03Z......

EXAMPLE−Example of AIRMET Tango issued for the Salt Lake CityFA area:SLCT WA 131445AIRMET TANGO UPDT 2 FOR TURB VALID UNTIL132100.AIRMET TURB...MT FROM 40NW HVR TO 50SE BIL TO 60E DLN TO60SW YQL TO 40NW HVRMOD TURB BLW 150. CONDS DVLPG 18−21Z.CONDS CONTG BYD 21Z THRU 03Z..AIRMET TURB....ID MT WY NV UT COFROM 100SE MLS TO 50SSW BFF TO 20SW BTYTO 40SW BAM TO 100SE MLSMOD TURB BTN FL310 AND FL410. CONDSCONTG BYD 21Z ENDG 21−00Z..AIRMET TURB...NV AZ NM CA AND CSTL WTRSFROM 100WSW ENI TO 40W BTY TO 40S LAS TO30ESE TBE TO INK TO ELP TO 50S TUS TO BZATO 20S MZB TO 150SW PYE TO 100WSW ENIMOD TURB BTWN FL210 AND FL380. CONDSCONTG BYD 21Z THRU 03Z.....

EXAMPLE−Example of AIRMET Zulu issued for the San FranciscoFA area:SFOZ WA 131445AIRMET ZULU UPDT 2 FOR ICE AND FRZLVL VALIDUNTIL 132100.NO SGFNT ICE EXP OUTSIDE OF CNVTV ACT..FRZLVL....RANGING FROM SFC−105 ACRS AREA MULT FRZLVL BLW 080 BOUNDED BY 40SE YDC−60NNW GEG−60SW MLP−30WSW BKE− 20SW BAM−70W BAM−40SW YKM−40E HUH− 40SE YDC SFC ALG 20NNW HUH−30SSE HUH−60S SEA 50NW LKV−60WNWOAL−30SW OAL 040 ALG 40W HUH−30W HUH−30NNW SEA−40N PDX−20NNW DSD 080 ALG 160NW FOT−80SW ONP−50SSW EUG 40SSE OED−50SSE CZQ−60E EHF−40WSW LAS....

3. Graphical AIRMETs (G−AIRMETs),found on the Aviation Weather Center webpage athttp://aviationweather.gov, are graphical forecasts ofen−route weather hazards valid at discrete times nomore than 3 hours apart for a period of up to 12 hoursinto the future (for example, 00, 03, 06, 09, and 12hours). Additional forecasts may be inserted during

the first 6 hours (for example, 01, 02, 04, and 05). 00hour represents the initial conditions, and thesubsequent graphics depict the area affected by theparticular hazard at that valid time. Forecasts valid at00 through 06 hours correspond to the text AIRMETbulletin. Forecasts valid at 06 through 12 hourscorrespond to the text bulletin outlook. G−AIRMETdepicts the following en route aviation weatherhazards:

(a) Instrument flight rule conditions (ceiling< 1000’ and/or surface visibility <3 miles)

(b) Mountain obscuration

(c) Icing

(d) Freezing level

(e) Turbulence

(f) Low level wind shear (LLWS)

(g) Strong surface winds

G−AIRMETs are snap shots at discrete time intervalsas defined above. The text AIRMET is the result ofthe production of the G−AIRMET but provided in atime smear for a 6hr valid period. G−AIRMETsprovide a higher forecast resolution than textAIRMET products. Since G−AIRMETs and textAIRMETs are created from the same forecast“production” process, there exists perfect consisten-cy between the two. Using the two together willprovide clarity of the area impacted by the weatherhazard and improve situational awareness anddecision making.

Interpolation of time periods between G−AIRMETvalid times: Users must keep in mind when using theG−AIRMET that if a 00 hour forecast shows nosignificant weather and a 03 hour forecast showshazardous weather, they must assume a change isoccurring during the period between the twoforecasts. It should be taken into consideration thatthe hazardous weather starts immediately after the 00hour forecast unless there is a defined initiation orending time for the hazardous weather. The samewould apply after the 03 hour forecast. The usershould assume the hazardous weather conditionis occurring between the snap shots unless informedotherwise. For example, if a 00 hour forecast showsno hazard, a 03 hour forecast shows the presence ofhazardous weather, and a 06 hour forecast shows nohazard, the user should assume the hazard exists fromthe 0001 hour to the 0559 hour time period.

AIM8/15/19


EXAMPLE−See FIG 7−1−6 for an example of the G−AIRMETgraphical product.

g. Watch Notification Messages

The Storm Prediction Center (SPC) in Norman, OK,issues Watch Notification Messages to provide anarea threat alert for forecast organized severethunderstorms that may produce tornadoes, largehail, and/or convective damaging winds within theCONUS. SPC issues three types of watch notificationmessages: Aviation Watch Notification Messages,Public Severe Thunderstorm Watch NotificationMessages, and Public Tornado Watch NotificationMessages.

It is important to note the difference between a SevereThunderstorm (or Tornado) Watch and a SevereThunderstorm (or Tornado) Warning. A watch meanssevere weather is possible during the next few hours,while a warning means that severe weather has beenobserved, or is expected within the hour. Only theSPC issues Severe Thunderstorm and TornadoWatches, while only NWS Weather Forecasts Officesissue Severe Thunderstorm and Tornado Warnings.

1. The Aviation Watch Notification Message.The Aviation Watch Notification Message product isan approximation of the area of the Public SevereThunderstorm Watch or Public Tornado Watch. Thearea may be defined as a rectangle or parallelogramusing VOR navigational aides as coordinates.

The Aviation Watch Notification Message wasformerly known as the Alert Severe Weather WatchBulletin (AWW). The NWS no longer uses that titleor acronym for this product. The NWS uses theacronym SAW for the Aviation Watch NotificationMessage, but retains AWW in the product header forprocessing by weather data systems.

EXAMPLE−Example of an Aviation Watch Notification Message:WWUS30 KWNS 271559SAW2SPC AWW 271559WW 568 TORNADO AR LA MS 271605Z - 280000ZAXIS..65 STATUTE MILES EAST AND WEST OF LINE..45ESE HEZ/NATCHEZ MS/ - 50N TUP/TUPELO MS/..AVIATION COORDS.. 55NM E/W /18WNW MCB - 60EMEM/HAIL SURFACE AND ALOFT..3 INCHES. WINDGUSTS..70 KNOTS. MAX TOPS TO 550. MEAN STORMMOTION VECTOR 26030. LAT...LON 31369169 34998991 34998762 31368948

THIS IS AN APPROXIMATION TO THE WATCH AREA.FOR A COMPLETE DEPICTION OF THE WATCH SEEWOUS64 KWNS FOR WOU2.

2. Public Severe Thunderstorm Watch Notifica-tion Messages describe areas of expected severethunderstorms. (Severe thunderstorm criteria are1-inch hail or larger and/or wind gusts of 50 knots [58mph] or greater). A Public Severe ThunderstormWatch Notification Message contains the areadescription and axis, the watch expiration time, adescription of hail size and thunderstorm wind gustsexpected, the definition of the watch, a call to actionstatement, a list of other valid watches, a briefdiscussion of meteorological reasoning and technicalinformation for the aviation community.

3. Public Tornado Watch Notification Messagesdescribe areas where the threat of tornadoes exists. APublic Tornado Watch Notification Message containsthe area description and axis, watch expiration time,the term “damaging tornadoes,” a description of thelargest hail size and strongest thunderstorm windgusts expected, the definition of the watch, a call toaction statement, a list of other valid watches, a briefdiscussion of meteorological reasoning and technicalinformation for the aviation community. SPC mayenhance a Public Tornado Watch NotificationMessage by using the words “THIS IS APARTICULARLY DANGEROUS SITUATION”when there is a likelihood of multiple strong (damageof EF2 or EF3) or violent (damage of EF4 or EF5)tornadoes.

4. Public severe thunderstorm and tornadowatch notification messages were formerly known asthe Severe Weather Watch Bulletins (WW). TheNWS no longer uses that title or acronym for thisproduct but retains WW in the product header forprocessing by weather data systems.

EXAMPLE−Example of a Public Tornado Watch Notification Message:WWUS20 KWNS 050550SEL2SPC WW 051750URGENT - IMMEDIATE BROADCAST REQUESTEDTORNADO WATCH NUMBER 243NWS STORM PREDICTION CENTER NORMAN OK1250 AM CDT MON MAY 5 2011THE NWS STORM PREDICTION CENTER HAS ISSUEDA*TORNADO WATCH FOR PORTIONS OFWESTERN AND CENTRAL ARKANSAS

AIM 8/15/19


SOUTHERN MISSOURIFAR EASTERN OKLAHOMA*EFFECTIVE THIS MONDAY MORNING FROM 1250AM UNTIL 600 AM CDT....THIS IS A PARTICULARLY DANGEROUS SITUA-TION...*PRIMARY THREATS INCLUDENUMEROUS INTENSE TORNADOES LIKELYNUMEROUS SIGNIFICANT DAMAGING WIND GUSTSTO 80 MPH LIKELYNUMEROUS VERY LARGE HAIL TO 4 INCHES INDIAMETER LIKELYTHE TORNADO WATCH AREA IS APPROXIMATELYALONG AND 100 STATUTE MILES EAST AND WEST OFA LINE FROM 15 MILES WEST NORTHWEST OF FORTLEONARD WOOD MISSOURI TO 45 MILES SOUTH-WEST OF HOT SPRINGS ARKANSAS. FOR ACOMPLETE DEPICTION OF THE WATCH SEE THEASSOCIATED WATCH OUTLINE UPDATE (WOUS64KWNS WOU2).REMEMBER...A TORNADO WATCH MEANS CONDI-TIONS ARE FAVORABLE FOR TORNADOES ANDSEVERE THUNDERSTORMS IN AND CLOSE TO THEWATCH AREA. PERSONS IN THESE AREAS SHOULDBE ON THE LOOKOUT FOR THREATENING WEATH-ER CONDITIONS AND LISTEN FOR LATERSTATEMENTS AND POSSIBLE WARNINGS.OTHER WATCH INFORMATION...THIS TORNADOWATCH REPLACES TORNADO WATCH NUMBER 237.WATCH NUMBER 237 WILL NOT BE IN EFFECTAFTER1250 AM CDT. CONTINUE...WW 239...WW 240...WW241...WW 242...DISCUSSION...SRN MO SQUALL LINE EXPECTED TOCONTINUE EWD...WHERE LONG/HOOKEDHODOGRAPHS SUGGEST THREAT FOR EMBEDDEDSUPERCELLS/POSSIBLE TORNADOES. FARTHERS...MORE WIDELY SCATTEREDSUPERCELLS WITH A THREAT FOR TORNADOESWILL PERSIST IN VERY STRONGLY DEEP SHEARED/LCL ENVIRONMENT IN AR.AVIATION...TORNADOES AND A FEW SEVERE THUN-DERSTORMS WITH HAIL SURFACE AND ALOFT TO 4INCHES. EXTREME TURBULENCE AND SURFACEWIND GUSTS TO 70 KNOTS. A FEW CUMULONIMBIWITH MAXIMUM TOPS TO 500. MEAN STORMMOTION VECTOR 26045.

5. Status reports are issued as needed to showprogress of storms and to delineate areas no longerunder the threat of severe storm activity. Cancellationbulletins are issued when it becomes evident that nosevere weather will develop or that storms havesubsided and are no longer severe.

h. Center Weather Advisories (CWAs)

1. CWAs are unscheduled inflight, flow control,air traffic, and air crew advisory. By nature of its shortlead time, the CWA is not a flight planning product.It is generally a nowcast for conditions beginningwithin the next two hours. CWAs will be issued:

(a) As a supplement to an existing SIGMET,Convective SIGMET or AIRMET.

(b) When an Inflight Advisory has not beenissued but observed or expected weather conditionsmeet SIGMET/AIRMET criteria based on currentpilot reports and reinforced by other sourcesof information about existing meteorological condi-tions.

(c) When observed or developing weatherconditions do not meet SIGMET, ConvectiveSIGMET, or AIRMET criteria; e.g., in terms ofintensity or area coverage, but current pilot reports orother weather information sources indicate thatexisting or anticipated meteorological phenomenawill adversely affect the safe flow of air traffic withinthe ARTCC area of responsibility.

2. The following example is a CWA issued fromthe Kansas City, Missouri, ARTCC. The “3” afterZKC in the first line denotes this CWA has beenissued for the third weather phenomena to occur forthe day. The “301” in the second line denotes thephenomena number again (3) and the issuancenumber (01) for this phenomena. The CWA wasissued at 2140Z and is valid until 2340Z.

EXAMPLE−ZKC3 CWA 032140ZKC CWA 301 VALID UNTIL 032340ISOLD SVR TSTM over KCOU MOVG SWWD10 KTS ETC.

7−1−7. Categorical Outlooks

a. Categorical outlook terms, describing generalceiling and visibility conditions for advancedplanning purposes are used only in area forecasts andare defined as follows:

1. LIFR (Low IFR). Ceiling less than 500 feetand/or visibility less than 1 mile.

2. IFR. Ceiling 500 to less than 1,000 feetand/or visibility 1 to less than 3 miles.

3. MVFR (Marginal VFR). Ceiling 1,000 to3,000 feet and/or visibility 3 to 5 miles inclusive.

AIM8/15/19


4. VFR. Ceiling greater than 3,000 feet andvisibility greater than 5 miles; includes sky clear.

b. The cause of LIFR, IFR, or MVFR is indicatedby either ceiling or visibility restrictions or both. Thecontraction “CIG” and/or weather and obstruction tovision symbols are used. If winds or gusts of 25 knotsor greater are forecast for the outlook period, the word“WIND” is also included for all categories includingVFR.

EXAMPLE−1. LIFR CIG−low IFR due to low ceiling.

2. IFR FG−IFR due to visibility restricted by fog.

3. MVFR CIG HZ FU−marginal VFR due to both ceilingand visibility restricted by haze and smoke.

4. IFR CIG RA WIND−IFR due to both low ceiling andvisibility restricted by rain; wind expected to be 25 knots orgreater.

7−1−8. Telephone Information BriefingService (TIBS) (Alaska Only)

a. TIBS, provided by FSS, is a system ofautomated telephone recordings of meteorologicaland aeronautical information available in Alaska.Based on the specific needs of each area, TIBSprovides route and/or area briefings in addition toairspace procedures and special announcementsconcerning aviation interests that may be available.Depending on user demand, other items may beprovided; for example, surface weather observations,terminal forecasts, wind and temperatures aloftforecasts, etc.

b. TIBS is not intended to be a substitute forspecialist−provided preflight briefings from FSS.TIBS is recommended as a preliminary briefing andoften will be valuable in helping you to make a “go”or “no go” decision.

c. Pilots are encouraged to utilize TIBS, which canbe accessed by dialing the FSS toll−free telephonenumber, 1−800−WX−BRIEF (992−7433) or specificpublished TIBS telephone numbers in certain areas.Consult the “FSS Telephone Numbers” section of theChart Supplement U.S. or the Chart SupplementAlaska or Pacific.

NOTE−A touch−tone telephone is necessary to fully utilize TIBS.

7−1−9. Transcribed Weather Broadcast(TWEB) (Alaska Only)

Equipment is provided in Alaska by whichmeteorological and aeronautical data are recorded ontapes and broadcast continuously over selected L/MFand VOR facilities. Broadcasts are made from a seriesof individual tape recordings, and changes, as theyoccur, are transcribed onto the tapes. The informationprovided varies depending on the type equipmentavailable. Generally, the broadcast contains asummary of adverse conditions, surface weatherobservations, pilot weather reports, and a densityaltitude statement (if applicable). At the discretion ofthe broadcast facility, recordings may also include asynopsis, winds aloft forecast, en route and terminalforecast data, and radar reports. At selected locations,telephone access to the TWEB has been provided(TEL−TWEB). Telephone numbers for this serviceare found in the Chart Supplement Alaska. Thesebroadcasts are made available primarily for preflightand inflight planning, and as such, should not beconsidered as a substitute for specialist−providedpreflight briefings.

7−1−10. Inflight Weather Broadcasts

a. Weather Advisory Broadcasts. ARTCCsbroadcast a Severe Weather Forecast Alert (AWW),Convective SIGMET, SIGMET, or CWA alert onceon all frequencies, except emergency, when any partof the area described is within 150 miles of theairspace under their jurisdiction. These broadcastscontain SIGMET or CWA (identification) and a briefdescription of the weather activity and general areaaffected.

EXAMPLE−1. Attention all aircraft, SIGMET Delta Three, from Mytonto Tuba City to Milford, severe turbulence and severe clearicing below one zero thousand feet. Expected to continuebeyond zero three zero zero zulu.

2. Attention all aircraft, convective SIGMET Two SevenEastern. From the vicinity of Elmira to Phillipsburg.Scattered embedded thunderstorms moving east at onezero knots. A few intense level five cells, maximum tops fourfive zero.

3. Attention all aircraft, Kansas City Center weatheradvisory one zero three. Numerous reports of moderate tosevere icing from eight to niner thousand feet in a three zeromile radius of St. Louis. Light or negative icing reportedfrom four thousand to one two thousand feet remainder ofKansas City Center area.

AIM 8/15/19


NOTE−1. Terminal control facilities have the option to limit theAWW, convective SIGMET, SIGMET, or CWA broadcast asfollows: local control and approach control positions mayopt to broadcast SIGMET or CWA alerts only when anypart of the area described is within 50 miles of the airspaceunder their jurisdiction.

2. In areas where HIWAS is available, ARTCC, TerminalATC, and FSS facilities no longer broadcast InflightWeather Advisories as described above in paragraph a. Seeparagraphs b1 and b2 below.

b. Hazardous Inflight Weather Advisory Ser-vice (HIWAS). HIWAS is an automated, continuousbroadcast of inflight weather advisories, provided byFSS over select VOR outlets, which include thefollowing weather products: AWW, SIGMET,Convective SIGMET, CWA, AIRMET (text [WA] orgraphical [G−AIRMET] products), and urgentPIREP. HIWAS is available throughout the contermi-nous United States as an additional source ofhazardous weather information. HIWAS does notreplace preflight or inflight weather briefings fromFSS. Pilots should call FSS if there are any questionsabout weather that is different than forecasted or if theHIWAS broadcast appears to be in error.

1. Where HIWAS is available, ARTCC andterminal ATC facilities will broadcast, upon receipt,

a HIWAS alert once on all frequencies, exceptemergency frequencies. Included in the broadcastwill be an alert announcement, frequency instruction,number, and type of advisory updated; for example,AWW, SIGMET, Convective SIGMET, or CWA.

EXAMPLE−Attention all aircraft. Hazardous weather information(SIGMET, Convective SIGMET, AIRMET (text [WA] orgraphical [G−AIRMET] product), Urgent PilotWeather Report [UUA], or Center Weather Advisory[CWA], Number or Numbers) for (geographical area)available on HIWAS or Flight Service frequencies.

2. Upon notification of an update to HIWAS,FSS will broadcast a HIWAS update announcementonce on all frequencies except emergency frequen-cies. Included in the broadcast will be the type ofadvisory updated; for example, AWW, SIGMET,Convective SIGMET, CWA, etc.

EXAMPLE−Attention all aircraft. Hazardous weather information for(geographical area) available from Flight Service.

3. HIWAS availability is notated with VORlistings in the Chart Supplement U.S., and is shownby symbols on IFR Enroute Low Altitude Charts andVFR Sectional Charts. The symbol depiction isidentified in the chart legend.

AIM8/15/19


FIG 7−1−6

G−AIRMET Graphical Product

AIM 8/15/19


7−1−11. Flight Information Services (FIS)

a. FIS. FIS is a method of disseminatingmeteorological (MET) and aeronautical information(AI) to displays in the cockpit in order to enhancepilot situational awareness, provide decision supporttools, and improve safety. FIS augments traditionalpilot voice communication with Flight ServiceStations (FSSs), ATC facilities, or Airline OperationsControl Centers (AOCCs). FIS is not intended toreplace traditional pilot and controller/flight servicespecialist/aircraft dispatcher preflight briefings orinflight voice communications. FIS, however, canprovide textual and graphical information that canhelp abbreviate and improve the usefulness of suchcommunications. FIS enhances pilot situationalawareness and improves safety.

1. Data link Service Providers (DLSP) - DLSPdeploy and maintain airborne, ground-based, and, insome cases, space-based infrastructure that supportsthe transmission of AI/MET information over one ormore physical links. DLSP may provide a free ofcharge or for-fee service that permits end users touplink and downlink AI/MET and other information.The following are examples of DLSP:

(a) FAA FIS-B. A ground-based broadcastservice provided through the ADS-B UniversalAccess Transceiver (UAT) network. The serviceprovides users with a 978 MHz data link capabilitywhen operating within range and line-of-sight of atransmitting ground station. FIS-B enables users ofproperly equipped aircraft to receive and display asuite of broadcast weather and aeronautical informa-tion products.

(b) Non-FAA FIS Systems. Several commer-cial vendors provide customers with FIS data overboth the aeronautical spectrum and on otherfrequencies using a variety of data link protocols.Services available from these providers vary greatlyand may include tier based subscriptions. Advance-ments in bandwidth technology permits preflight aswell as inflight access to the same MET and AIinformation available on the ground. Pilots andoperators using non-FAA FIS for MET and AIinformation should be knowledgeable regarding theweather services being provided as some commercialvendors may be repackaging NWS sourced weather,while other commercial vendors may alter theweather information to produce vendor−tailored orvendor−specific weather reports and forecasts.

2. Three Data Link Modes. There are three datalink modes that may be used for transmitting AI andMET information to aircraft. The intended use of theAI and/or MET information will determine the mostappropriate data link service.

(a) Broadcast Mode: A one-way interactionin which AI and/or MET updates or changesapplicable to a designated geographic area arecontinuously transmitted (or transmitted at repeatedperiodic intervals) to all aircraft capable of receivingthe broadcast within the service volume defined bythe system network architecture.

(b) Contract/Demand Mode: A two-wayinteraction in which AI and/or MET information istransmitted to an aircraft in response to a specificrequest.

(c) Contract/Update Mode: A two-way inter-action that is an extension of the Demand Mode.Initial AI and/or MET report(s) are sent to an aircraftand subsequent updates or changes to the AI and/orMET information that meet the contract criteria areautomatically or manually sent to an aircraft.

3. To ensure airman compliance with FederalAviation Regulations, manufacturer’s operatingmanuals should remind airmen to contact ATCcontrollers, FSS specialists, operator dispatchers, orairline operations control centers for general andmission critical aviation weather information and/orNAS status conditions (such as NOTAMs, SpecialUse Airspace status, and other government flightinformation). If FIS products are systemicallymodified (for example, are displayed as abbreviatedplain text and/or graphical depictions), the modifica-tion process and limitations of the resultant productshould be clearly described in the vendor’s userguidance.

4. Operational Use of FIS. Regardless of thetype of FIS system being used, several factors mustbe considered when using FIS:

(a) Before using FIS for inflight operations,pilots and other flight crewmembers should becomefamiliar with the operation of the FIS system to beused, the airborne equipment to be used, including itssystem architecture, airborne system components,coverage service volume and other limitations of theparticular system, modes of operation and indicationsof various system failures. Users should also befamiliar with the specific content and format of theservices available from the FIS provider(s). Sources

AIM8/15/19


of information that may provide this specificguidance include manufacturer’s manuals, trainingprograms, and reference guides.

(b) FIS should not serve as the sole source ofaviation weather and other operational information.ATC, FSSs, and, if applicable, AOCC VHF/HF voiceremain as a redundant method of communicatingaviation weather, NOTAMs, and other operationalinformation to aircraft in flight. FIS augments thesetraditional ATC/FSS/AOCC services and, for someproducts, offers the advantage of being displayed asgraphical information. By using FIS for orientation,the usefulness of information received fromconventional means may be enhanced. For example,FIS may alert the pilot to specific areas of concernthat will more accurately focus requests made to FSSor AOCC for inflight updates or similar queries madeto ATC.

(c) The airspace and aeronautical environ-ment is constantly changing. These changes occurquickly and without warning. Critical operationaldecisions should be based on use of the most currentand appropriate data available. When differencesexist between FIS and information obtained by voicecommunication with ATC, FSS, and/or AOCC (ifapplicable), pilots are cautioned to use the mostrecent data from the most authoritative source.

(d) FIS aviation weather products (forexample, graphical ground−based radar precipitationdepictions) are not appropriate for tactical (typicaltimeframe of less than 3 minutes) avoidance of severeweather such as negotiating a path through a weatherhazard area. FIS supports strategic (typical timeframeof 20 minutes or more) weather decisionmaking suchas route selection to avoid a weather hazard area in itsentirety. The misuse of information beyond itsapplicability may place the pilot and aircraft injeopardy. In addition, FIS should never be used in lieuof an individual preflight weather and flight planningbriefing.

(e) DLSP offer numerous MET and AIproducts with information that can be layered on topof each other. Pilots need to be aware that too muchinformation can have a negative effect on theircognitive work load. Pilots need to manage theamount of information to a level that offers the mostpertinent information to that specific flight withoutcreating a cockpit distraction. Pilots may need to

adjust the amount of information based on numerousfactors including, but not limited to, the phase offlight, single pilot operation, autopilot availability,class of airspace, and the weather conditionsencountered.

(f) FIS NOTAM products, including Tempo-rary Flight Restriction (TFR) information, areadvisory−use information and are intended forsituational awareness purposes only. Cockpit dis-plays of this information are not appropriate fortactical navigation − pilots should stay clear of anygeographic area displayed as a TFR NOTAM. Pilotsshould contact FSSs and/or ATC while en route toobtain updated information and to verify the cockpitdisplay of NOTAM information.

(g) FIS supports better pilot decisionmakingby increasing situational awareness. Better decision−making is based on using information from a varietyof sources. In addition to FIS, pilots should takeadvantage of other weather/NAS status sources,including, briefings from Flight Service Stations,data from other air traffic control facilities, airlineoperation control centers, pilot reports, as well astheir own observations.

(h) FAA’s Flight Information Service−Broad-cast (FIS−B).

(1) FIS−B is a ground−based broadcastservice provided through the FAA’s AutomaticDependent Surveillance–Broadcast (ADS−B) Ser-vices Universal Access Transceiver (UAT) network.The service provides users with a 978 MHz data linkcapability when operating within range and line−of−sight of a transmitting ground station. FIS−B enablesusers of properly−equipped aircraft to receive anddisplay a suite of broadcast weather and aeronauticalinformation products.

(2) The following list represents the initialsuite of text and graphical products available throughFIS−B and provided free−of−charge. Detailedinformation concerning FIS−B meteorological prod-ucts can be found in Advisory Circular 00−45,Aviation Weather Services, and AC 00-63, Use ofCockpit Displays of Digital Weather and Aeronauti-cal Information. Information on Special UseAirspace (SUA), Temporary Flight Restriction(TFR), and Notice to Airmen (NOTAM) products canbe found in Chapters 3, 4 and 5 of this manual.

AIM 8/15/19


[a] Text: Aviation Routine WeatherReport (METAR) and Special Aviation Report(SPECI);

[b] Text: Pilot Weather Report (PIREP);

[c] Text: Winds and Temperatures Aloft;

[d] Text: Terminal Aerodrome Forecast(TAF) and amendments;

[e] Text: Notice to Airmen (NOTAM)Distant and Flight Data Center;

[f] Text/Graphic: Airmen’s Meteoro-logical Conditions (AIRMET);

[g] Text/Graphic: Significant Meteoro-logical Conditions (SIGMET);

[h] Text/Graphic: Convective SIG-MET;

[i] Text/Graphic: Special Use Airspace(SUA);

[j] Text/Graphic: Temporary Flight Re-striction (TFR) NOTAM; and

[k] Graphic: NEXRAD Composite Re-flectivity Products (Regional and National).

(3) Users of FIS−B should familiarizethemselves with the operational characteristics andlimitations of the system, including: system architec-ture; service environment; product lifecycles; modesof operation; and indications of system failure.

(4) FIS−B products are updated andtransmitted at specific intervals based primarily onproduct issuance criteria. Update intervals aredefined as the rate at which the product data isavailable from the source for transmission. Transmis-sion intervals are defined as the amount of time withinwhich a new or updated product transmission must becompleted and/or the rate or repetition interval atwhich the product is rebroadcast. Update andtransmission intervals for each product are providedin TBL 7−1−1.

(5) Where applicable, FIS−B productsinclude a look−ahead range expressed in nauticalmiles (NM) for three service domains: AirportSurface; Terminal Airspace; and Enroute/Gulf−of−Mexico (GOMEX). TBL 7−1−2 provides servicedomain availability and look−ahead ranging for eachFIS−B product.

(6) Prior to using this capability, usersshould familiarize themselves with the operation ofFIS−B avionics by referencing the applicable User’sGuides. Guidance concerning the interpretation ofinformation displayed should be obtained from theappropriate avionics manufacturer.

(7) FIS−B malfunctions not attributed toaircraft system failures or covered by active NOTAMshould be reported by radio or telephone to the nearestFSS facility.

b. Non−FAA FIS Systems. Several commercialvendors also provide customers with FIS data overboth the aeronautical spectrum and on otherfrequencies using a variety of data link protocols. Insome cases, the vendors provide only the communi-cations system that carries customer messages, suchas the Aircraft Communications Addressing andReporting System (ACARS) used by many air carrierand other operators.

1. Operators using non−FAA FIS data forinflight weather and other operational informationshould ensure that the products used conform toFAA/NWS standards. Specifically, aviation weatherand NAS status information should meet thefollowing criteria:

(a) The products should be either FAA/NWS“accepted” aviation weather reports or products, orbased on FAA/NWS accepted aviation weatherreports or products. If products are used which do notmeet this criteria, they should be so identified. Theoperator must determine the applicability of suchproducts to their particular flight operations.

(b) In the case of a weather product which isthe result of the application of a process which altersthe form, function or content of the base FAA/NWSaccepted weather product(s), that process, and anylimitations to the application of the resultant product,should be described in the vendor’s user guidancematerial.

2. An example would be a NEXRAD radarcomposite/mosaic map, which has been modified bychanging the scaling resolution. The methodology ofassigning reflectivity values to the resultant imagecomponents should be described in the vendor’sguidance material to ensure that the user canaccurately interpret the displayed data.

AIM8/15/19


TBL 7−1−1

FIS−B Over UAT Product Update and Transmission Intervals

Product FIS-B Over UAT Service

Update Intervals1FIS-B Service

Transmission

Intervals2

AIRMET As Available 5 minutes

Convective SIGMET As Available 5 minutes

METARs/SPECIs 1 minute/As Available 5 minutes

NEXRAD Composite Reflectivity (CONUS) 15 minutes 15 minutes

NEXRAD Composite Reflectivity (Regional) 5 minutes 2.5 minutes

NOTAMs-D/FDC/TFR As Available 10 minutes

PIREP As Available 10 minutes

SIGMET As Available 5 minutes

SUA Status As Available 10 minutes

TAF/AMEND 8 Hours/As Available 10 minutes

Temperatures Aloft 12 Hours 10 minutes

Winds Aloft 12 Hours 10 minutes

1 The Update Interval is the rate at which the product data is available from the source.

2 The Transmission Interval is the amount of time within which a new or updated product transmission must be completedand the rate or repetition interval at which the product is rebroadcast.

AIM 8/15/19


TBL 7−1−2

Product Parameters for Low/Medium/High Altitude Tier Radios

Product Surface Radios Low Altitude Tier Medium AltitudeTier

High Altitude Tier

CONUS NEXRAD N/A CONUS NEXRADnot provided

CONUS NEXRADimagery

CONUS NEXRADimagery

Winds & TempsAloft

500 NM look−aheadrange



1,000 NM look−ahead range

METAR 100 NM look−aheadrange



CONUS: CONUSClass B & C airportMETARs and 500NM look−aheadrange

Outside of CONUS:500 NM look-aheadrange

TAF 100 NM look−aheadrange



CONUS: CONUSClass B & C airportTAFs and 500 NMlook−ahead range

Outside of CONUS:500 NM look-aheadrange

AIRMET, SIGMET,PIREP, and SUA/SAA

100 NM look−aheadrange. PIREP/SUA/SAA is N/A.




Regional NEXRAD 150 NM look−aheadrange




NOTAMs D, FDC,and TFR





7−1−12. Weather Observing Programs

a. Manual Observations. With only a fewexceptions, these reports are from airport locationsstaffed by FAA personnel who manually observe,perform calculations, and enter these observationsinto the (WMSCR) communication system. Theformat and coding of these observations arecontained in Paragraph 7−1−30 , Key to AviationRoutine Weather Report (METAR) and AerodromeForecasts (TAF).

b. Automated Weather Observing System(AWOS).

1. Automated weather reporting systems areincreasingly being installed at airports. Thesesystems consist of various sensors, a processor, acomputer-generated voice subsystem, and a transmit-

ter to broadcast local, minute-by-minute weather datadirectly to the pilot.

NOTE−When the barometric pressure exceeds 31.00 inches Hg.,see Paragraph 7−2−2 , Procedures, for the altimetersetting procedures.

2. The AWOS observations will include theprefix “AUTO” to indicate that the data are derivedfrom an automated system. Some AWOS locationswill be augmented by certified observers who willprovide weather and obstruction to vision informa-tion in the remarks of the report when the reportedvisibility is less than 7 miles. These sites, along withthe hours of augmentation, are to be published in theChart Supplement U.S. Augmentation is identified inthe observation as “OBSERVER WEATHER.” TheAWOS wind speed, direction and gusts, temperature,

AIM8/15/19


dew point, and altimeter setting are exactly the sameas for manual observations. The AWOS will alsoreport density altitude when it exceeds the fieldelevation by more than 1,000 feet. The reportedvisibility is derived from a sensor near the touchdownof the primary instrument runway. The visibilitysensor output is converted to a visibility value usinga 10−minute harmonic average. The reported skycondition/ceiling is derived from the ceilometerlocated next to the visibility sensor. The AWOSalgorithm integrates the last 30 minutes of ceilometerdata to derive cloud layers and heights. This outputmay also differ from the observer sky condition inthat the AWOS is totally dependent upon the cloudadvection over the sensor site.

3. These real-time systems are operationallyclassified into nine basic levels:

(a) AWOS−A only reports altimeter setting;

NOTE−Any other information is advisory only.

(b) AWOS−AV reports altimeter and visibili-ty;

NOTE−Any other information is advisory only.

(c) AWOS−l usually reports altimeter setting,wind data, temperature, dew point, and densityaltitude;

(d) AWOS−2 provides the information pro-vided by AWOS−l plus visibility; and

(e) AWOS−3 provides the information pro-vided by AWOS−2 plus cloud/ceiling data.

(f) AWOS− 3P provides reports the same asthe AWOS 3 system, plus a precipitation identifica-tion sensor.

(g) AWOS− 3PT reports the same as theAWOS 3P System, plus thunderstorm/lightningreporting capability.

(h) AWOS− 3T reports the same as AWOS 3system and includes a thunderstorm/lightningreporting capability.

(i) AWOS− 4 reports the same as the AWOS3 system, plus precipitation occurrence, type andaccumulation, freezing rain, thunderstorm, andrunway surface sensors.

4. The information is transmitted over a discreteVHF radio frequency or the voice portion of a local

NAVAID. AWOS transmissions on a discrete VHFradio frequency are engineered to be receivable to amaximum of 25 NM from the AWOS site and amaximum altitude of 10,000 feet AGL. At manylocations, AWOS signals may be received on thesurface of the airport, but local conditions may limitthe maximum AWOS reception distance and/oraltitude. The system transmits a 20 to 30 secondweather message updated each minute. Pilots shouldmonitor the designated frequency for the automatedweather broadcast. A description of the broadcast iscontained in subparagraph c. There is no two-waycommunication capability. Most AWOS sites alsohave a dial-up capability so that the minute-by-minute weather messages can be accessed viatelephone.

5. AWOS information (system level, frequency,phone number, etc.) concerning specific locations ispublished, as the systems become operational, in theChart Supplement U.S., and where applicable, onpublished Instrument Approach Procedures. Selectedindividual systems may be incorporated intonationwide data collection and dissemination net-works in the future.

c. AWOS Broadcasts. Computer-generatedvoice is used in AWOS to automate the broadcast ofthe minute-by-minute weather observations. Inaddition, some systems are configured to permit theaddition of an operator-generated voice message;e.g., weather remarks following the automatedparameters. The phraseology used generally followsthat used for other weather broadcasts. Following areexplanations and examples of the exceptions.

1. Location and Time. The location/name andthe phrase “AUTOMATED WEATHER OBSERVA-TION,” followed by the time are announced.

(a) If the airport’s specific location isincluded in the airport’s name, the airport’s name isannounced.

EXAMPLE−“Bremerton National Airport automated weather observa-tion, one four five six zulu;”“Ravenswood Jackson County Airport automated weatherobservation, one four five six zulu.”

(b) If the airport’s specific location is notincluded in the airport’s name, the location isannounced followed by the airport’s name.

AIM 8/15/19


EXAMPLE−“Sault Ste. Marie, Chippewa County International Airportautomated weather observation;”“Sandusky, Cowley Field automated weather observa-tion.”

(c) The word “TEST” is added following“OBSERVATION” when the system is not incommissioned status.

EXAMPLE−“Bremerton National Airport automated weather observa-tion test, one four five six zulu.”

(d) The phrase “TEMPORARILY INOPER-ATIVE” is added when the system is inoperative.

EXAMPLE−“Bremerton National Airport automated weather observ-ing system temporarily inoperative.”

2. Visibility.

(a) The lowest reportable visibility value inAWOS is “less than 1/4.” It is announced as“VISIBILITY LESS THAN ONE QUARTER.”

(b) A sensor for determining visibility is notincluded in some AWOS. In these systems, visibilityis not announced. “VISIBILITY MISSING” isannounced only if the system is configured with avisibility sensor and visibility information is notavailable.

3. Weather. In the future, some AWOSs are tobe configured to determine the occurrence ofprecipitation. However, the type and intensity maynot always be determined. In these systems, the word“PRECIPITATION” will be announced if precipita-tion is occurring, but the type and intensity are notdetermined.

4. Ceiling and Sky Cover.

(a) Ceiling is announced as either “CEIL-ING” or “INDEFINITE CEILING.” With theexception of indefinite ceilings, all automated ceilingheights are measured.

EXAMPLE−“Bremerton National Airport automated weather observa-tion, one four five six zulu. Ceiling two thousand overcast;”

“Bremerton National Airport automated weather observa-tion, one four five six zulu. Indefinite ceiling two hundred,sky obscured.”

(b) The word “Clear” is not used in AWOSdue to limitations in the height ranges of the sensors.

No clouds detected is announced as “NO CLOUDSBELOW XXX” or, in newer systems as “CLEARBELOW XXX” (where XXX is the range limit of thesensor).

EXAMPLE−“No clouds below one two thousand.”“Clear below one two thousand.”

(c) A sensor for determining ceiling and skycover is not included in some AWOS. In thesesystems, ceiling and sky cover are not announced.“SKY CONDITION MISSING” is announced only ifthe system is configured with a ceilometer and theceiling and sky cover information is not available.

5. Remarks. If remarks are included in theobservation, the word “REMARKS” is announcedfollowing the altimeter setting.

(a) Automated “Remarks.”

(1) Density Altitude.

(2) Variable Visibility.

(3) Variable Wind Direction.

(b) Manual Input Remarks. Manual inputremarks are prefaced with the phrase “OBSERVERWEATHER.” As a general rule the manual remarksare limited to:

(1) Type and intensity of precipitation.

(2) Thunderstorms and direction; and

(3) Obstructions to vision when the visibili-ty is 3 miles or less.

EXAMPLE−“Remarks ... density altitude, two thousand five hundred ...visibility variable between one and two ... wind directionvariable between two four zero and three one zero...observed weather ... thunderstorm moderate rainshowers and fog ... thunderstorm overhead.”

(c) If an automated parameter is “missing”and no manual input for that parameter is available,the parameter is announced as “MISSING.” Forexample, a report with the dew point “missing” andno manual input available, would be announced asfollows:

EXAMPLE−“Ceiling one thousand overcast ... visibility three ...precipitation ... temperature three zero, dew point missing... wind calm ... altimeter three zero zero one.”

(d) “REMARKS” are announced in thefollowing order of priority:

AIM8/15/19


(1) Automated “REMARKS.”

[a] Density Altitude.

[b] Variable Visibility.

[c] Variable Wind Direction.

(2) Manual Input “REMARKS.”

[a] Sky Condition.

[b] Visibility.

[c] Weather and Obstructions to Vision.

[d] Temperature.

[e] Dew Point.

[f] Wind; and

[g] Altimeter Setting.

EXAMPLE−“Remarks ... density altitude, two thousand five hundred ...visibility variable between one and two ... wind directionvariable between two four zero and three one zero ...observer ceiling estimated two thousand broken ...observer temperature two, dew point minus five.”

d. Automated Surface Observing System(ASOS)/Automated Weather Observing System(AWOS) The ASOS/AWOS is the primary surfaceweather observing system of the U.S. (See Key toDecode an ASOS/AWOS (METAR) Observation,FIG 7−1−7 and FIG 7−1−8.) The program to installand operate these systems throughout the U.S. is ajoint effort of the NWS, the FAA and the Departmentof Defense. ASOS/AWOS is designed to supportaviation operations and weather forecast activities.The ASOS/AWOS will provide continuous minute-by-minute observations and perform the basicobserving functions necessary to generate an aviationroutine weather report (METAR) and other aviationweather information. The information may betransmitted over a discrete VHF radio frequency orthe voice portion of a local NAVAID. ASOS/AWOStransmissions on a discrete VHF radio frequency areengineered to be receivable to a maximum of 25 NMfrom the ASOS/AWOS site and a maximum altitudeof 10,000 feet AGL. At many locations, ASOS/AWOS signals may be received on the surface of theairport, but local conditions may limit the maximumreception distance and/or altitude. While theautomated system and the human may differ in theirmethods of data collection and interpretation, bothproduce an observation quite similar in form and

content. For the “objective” elements such aspressure, ambient temperature, dew point tempera-ture, wind, and precipitation accumulation, both theautomated system and the observer use a fixedlocation and time-averaging technique. The quantita-tive differences between the observer and theautomated observation of these elements arenegligible. For the “subjective” elements, however,observers use a fixed time, spatial averagingtechnique to describe the visual elements (skycondition, visibility and present weather), while theautomated systems use a fixed location, timeaveraging technique. Although this is a fundamentalchange, the manual and automated techniques yieldremarkably similar results within the limits of theirrespective capabilities.

1. System Description.

(a) The ASOS/AWOS at each airport locationconsists of four main components:

(1) Individual weather sensors.

(2) Data collection and processing units.

(3) Peripherals and displays.

(b) The ASOS/AWOS sensors perform thebasic function of data acquisition. They continuouslysample and measure the ambient environment, deriveraw sensor data and make them available to thecollection and processing units.

2. Every ASOS/AWOS will contain thefollowing basic set of sensors:

(a) Cloud height indicator (one or possiblythree).

(b) Visibility sensor (one or possibly three).

(c) Precipitation identification sensor.

(d) Freezing rain sensor (at select sites).

(e) Pressure sensors (two sensors at smallairports; three sensors at large airports).

(f) Ambient temperature/Dew point tempera-ture sensor.

(g) Anemometer (wind direction and speedsensor).

(h) Rainfall accumulation sensor.

(i) Automated Lightning Detection and Re-porting System (ALDARS) (excluding Alaska andPacific Island sites).

AIM 8/15/19


3. The ASOS/AWOS data outlets include:

(a) Those necessary for on-site airport users.

(b) National communications networks.

(c) Computer-generated voice (availablethrough FAA radio broadcast to pilots, and dial-intelephone line).

NOTE−Wind direction broadcast over FAA radios is in referenceto magnetic north.

4. An ASOS/AWOS report without humanintervention will contain only that weather datacapable of being reported automatically. Themodifier for this METAR report is “AUTO.” When

an observer augments or backs−up an ASOS/AWOSsite, the “AUTO” modifier disappears.

5. There are two types of automated stations,AO1 for automated weather reporting stationswithout a precipitation discriminator, and AO2 forautomated stations with a precipitation discriminator.As appropriate, “AO1” and “AO2” must appear inremarks. (A precipitation discriminator can deter-mine the difference between liquid andfrozen/freezing precipitation).

NOTE−To decode an ASOS/AWOS report, refer to FIG 7−1−7 andFIG 7−1−8.

REFERENCE−A complete explanation of METAR terminology is located in AIM,Paragraph 7−1−30 , Key to Aerodrome Forecast (TAF) and AviationRoutine Weather Report (METAR).

AIM8/15/19


FIG 7−1−7

Key to Decode an ASOS/AWOS (METAR) Observation (Front)

AIM 8/15/19


FIG 7−1−8

Key to Decode an ASOS/AWOS (METAR) Observation (Back)

AIM8/15/19


e. TBL 7−1−3 contains a comparison of weatherobserving programs and the elements reported.

f. Service Standards. During 1995, a govern-ment/industry team worked to comprehensivelyreassess the requirements for surface observations atthe nation’s airports. That work resulted in agreementon a set of service standards, and the FAA and NWSASOS sites to which the standards would apply. Theterm “Service Standards” refers to the level of detailin weather observation. The service standards consistof four different levels of service (A, B, C, and D) asdescribed below. Specific observational elementsincluded in each service level are listed inTBL 7−1−4.

1. Service Level D defines the minimumacceptable level of service. It is a completelyautomated service in which the ASOS/AWOSobservation will constitute the entire observation,i.e., no additional weather information is added by ahuman observer. This service is referred to as a standalone D site.

2. Service Level C is a service in which thehuman observer, usually an air traffic controller,augments or adds information to the automatedobservation. Service Level C also includes backup ofASOS/AWOS elements in the event of an ASOS/AWOS malfunction or an unrepresentativeASOS/AWOS report. In backup, the human observer

inserts the correct or missing value for the automatedASOS/AWOS elements. This service is provided byair traffic controllers under the Limited AviationWeather Reporting Station (LAWRS) process, FSSand NWS observers, and, at selected sites,Non−Federal Observation Program observers.

Two categories of airports require detail beyondService Level C in order to enhance air traffic controlefficiency and increase system capacity. Services atthese airports are typically provided by contractweather observers, NWS observers, and, at somelocations, FSS observers.

3. Service Level B is a service in which weatherobservations consist of all elements provided underService Level C, plus augmentation of additional databeyond the capability of the ASOS/AWOS. Thiscategory of airports includes smaller hubs or specialairports in other ways that have worse than averagebad weather operations for thunderstorms and/orfreezing/frozen precipitation, and/or that are remoteairports.

4. Service Level A, the highest and mostdemanding category, includes all the data reported inService Standard B, plus additional requirements asspecified. Service Level A covers major aviationhubs and/or high volume traffic airports with averageor worse weather.

TBL 7−1−3

Weather Observing Programs

Ele

men

t

Rep

orte

d

Win

d

Vis

ibili

ty

Tem

pera

ture

Dew

Poi

nt

Alt

imet

er

Den

sity

Alt

imet

er

Clo

ud/C

eilin

g

Pre

cipi

tati

on

Iden

tifi

cati

on

Thu

nder

stor

m/

Lig

htni

ng

Pre

cipi

tati

onO

ccur

renc

e

Rai

nfal

lA

ccum

ulat

ion

Run

way

Sur

face

Con

diti

on

Fre

ezin

g R

ain

Occ

urre

nce

Rem

arks

Type

ASOS X X X X X X X X X X

AWOS−A X

AWOS−A/V X X

AWOS−1 X X X X

AWOS−2 X X X X X

AWOS−3 X X X X X X

AWOS−3P X X X X X X X

AWOS−3T X X X X X X X

AWOS−3P/T X X X X X X X X

AWOS−4 X X X X X X X X X X X X

Manual X X X X X X X

REFERENCE− FAA Order JO 7900.5, Surface Weather Observing, for element reporting.

AIM 8/15/19


TBL 7−1−4

SERVICE LEVEL AService Level A consists of all the elements ofService Levels B, C and D plus the elementslisted to the right, if observed.

10 minute longline RVR at precedented sites oradditional visibility increments of 1/8, 1/16 and 0

Sector visibilityVariable sky conditionCloud layers above 12,000 feet and cloud typesWidespread dust, sand and other obscurationsVolcanic eruptions

SERVICE LEVEL BService Level B consists of all the elements ofService Levels C and D plus the elements listed tothe right, if observed.

Longline RVR at precedented sites(may be instantaneous readout)

Freezing drizzle versus freezing rainIce pelletsSnow depth & snow increasing rapidly remarksThunderstorm and lightning location remarksObserved significant weather not at the station

remarks

SERVICE LEVEL CService Level C consists of all the elements of ServiceLevel D plus augmentation and backup by a humanobserver or an air traffic control specialist on locationnearby. Backup consists of inserting the correct value ifthe system malfunctions or is unrepresentative.Augmentation consists of adding the elements listed tothe right, if observed. During hours that the observingfacility is closed, the site reverts to Service Level D.

ThunderstormsTornadoesHailVirgaVolcanic ashTower visibilityOperationally significant remarks as deemed

appropriate by the observer

SERVICE LEVEL DThis level of service consists of an ASOS or AWOScontinually measuring the atmosphere at a point near therunway. The ASOS or AWOS senses and measures theweather parameters listed to the right.

WindVisibilityPrecipitation/Obstruction to visionCloud heightSky coverTemperatureDew pointAltimeter

7−1−13. Weather Radar Services

a. The National Weather Service operates anetwork of radar sites for detecting coverage,intensity, and movement of precipitation. Thenetwork is supplemented by FAA and DOD radarsites in the western sections of the country. Localwarning radar sites augment the network by operatingon an as needed basis to support warning and forecastprograms.

b. Scheduled radar observations are taken hourlyand transmitted in alpha-numeric format on weathertelecommunications circuits for flight planningpurposes. Under certain conditions, special radarreports are issued in addition to the hourly

transmittals. Data contained in the reports are alsocollected by the National Center for EnvironmentalPrediction and used to prepare national radarsummary charts for dissemination on facsimilecircuits.

c. A clear radar display (no echoes) does not meanthat there is no significant weather within thecoverage of the radar site. Clouds and fog are notdetected by the radar. However, when echoes arepresent, turbulence can be implied by the intensity ofthe precipitation, and icing is implied by the presenceof the precipitation at temperatures at or below zerodegrees Celsius. Used in conjunction with otherweather products, radar provides invaluable informa-tion for weather avoidance and flight planning.

AIM8/15/19


FIG 7−1−9

NEXRAD Coverage

AIM 8/15/19


FIG 7−1−10

NEXRAD Coverage

AIM8/15/19


FIG 7−1−11

NEXRAD Coverage

AIM 8/15/19


d. All En Route Flight Advisory Service facilitiesand FSSs have equipment to directly access the radardisplays from the individual weather radar sites.Specialists at these locations are trained to interpretthe display for pilot briefing and inflight advisoryservices. The Center Weather Service Units located inARTCCs also have access to weather radar displaysand provide support to all air traffic facilities withintheir center’s area.

e. Additional information on weather radarproducts and services can be found in AC 00−45,Aviation Weather Services.

REFERENCE−Pilot/Controller Glossary Term− Precipitation Radar WeatherDescriptions.AIM, Paragraph 7−1−28 , ThunderstormsChart Supplement U.S., Charts, NWS Upper Air Observing Stations andWeather Network for the location of specific radar sites.

7−1−14. ATC Inflight Weather AvoidanceAssistance

a. ATC Radar Weather Display.

1. ATC radars are able to display areas ofprecipitation by sending out a beam of radio energythat is reflected back to the radar antenna when itstrikes an object or moisture which may be in the formof rain drops, hail, or snow. The larger the object is,or the more dense its reflective surface, the strongerthe return will be presented. Radar weatherprocessors indicate the intensity of reflective returnsin terms of decibels (dBZ). ATC systems cannotdetect the presence or absence of clouds. The ATCsystems can often determine the intensity of aprecipitation area, but the specific character of thatarea (snow, rain, hail, VIRGA, etc.) cannot bedetermined. For this reason, ATC refers to allweather areas displayed on ATC radar scopes as“precipitation.”

2. All ATC facilities using radar weatherprocessors with the ability to determine precipitationintensity, will describe the intensity to pilots as:

(a) “LIGHT” (< 26 dBZ)

(b) “MODERATE” (26 to 40 dBZ)

(c) “HEAVY” (> 40 to 50 dBZ)

(d) “EXTREME” (> 50 dBZ)

NOTE−En route ATC radar’s Weather and Radar Processor(WARP) does not display light precipitation intensity.

3. ATC facilities that, due to equipmentlimitations, cannot display the intensity levels ofprecipitation, will describe the location of theprecipitation area by geographic position, or positionrelative to the aircraft. Since the intensity level is notavailable, the controller will state “INTENSITYUNKNOWN.”

4. ARTCC facilities normally use a Weather andRadar Processor (WARP) to display a mosaic of dataobtained from multiple NEXRAD sites. There is atime delay between actual conditions and thosedisplayed to the controller. For example, theprecipitation data on the ARTCC controller’s displaycould be up to 6 minutes old. When the WARP is notavailable, a second system, the narrowband Air RouteSurveillance Radar (ARSR) can display two distinctlevels of precipitation intensity that will be describedto pilots as “MODERATE” (30 to 40 dBZ) and“HEAVY TO EXTREME” ( > 40 dBZ ). The WARPprocessor is only used in ARTCC facilities.

5. ATC radar is not able to detect turbulence.Generally, turbulence can be expected to occur as therate of rainfall or intensity of precipitation increases.Turbulence associated with greater rates of rainfall/precipitation will normally be more severe than anyassociated with lesser rates of rainfall/precipitation.Turbulence should be expected to occur nearconvective activity, even in clear air. Thunderstormsare a form of convective activity that imply severe orgreater turbulence. Operation within 20 miles ofthunderstorms should be approached with greatcaution, as the severity of turbulence can be markedlygreater than the precipitation intensity might indicate.

b. Weather Avoidance Assistance.

1. To the extent possible, controllers will issuepertinent information on weather or chaff areas andassist pilots in avoiding such areas when requested.Pilots should respond to a weather advisory by eitheracknowledging the advisory or by acknowledging theadvisory and requesting an alternative course ofaction as follows:

(a) Request to deviate off course by stating aheading or degrees, direction of deviation, andapproximate number of miles. In this case, when therequested deviation is approved, navigation is at thepilot’s prerogative, but must maintain the altitudeassigned, and remain within the lateral restrictionsissued by ATC.

AIM8/15/19


(b) An approval for lateral deviation autho-rizes the pilot to maneuver left or right within thelimits specified in the clearance.

NOTE−1. It is often necessary for ATC to restrict the amount oflateral deviation (“twenty degrees right,” “up to fifteendegrees left,” “up to ten degrees left or right of course”).

2. The term “when able, proceed direct,” in an ATCweather deviation clearance, refers to the pilot’s ability toremain clear of the weather when returning tocourse/route.

(c) Request a new route to avoid the affectedarea.

(d) Request a change of altitude.

(e) Request radar vectors around the affectedareas.

2. For obvious reasons of safety, an IFR pilotmust not deviate from the course or altitude or flightlevel without a proper ATC clearance. When weatherconditions encountered are so severe that animmediate deviation is determined to be necessaryand time will not permit approval by ATC, the pilot’semergency authority may be exercised.

3. When the pilot requests clearance for a routedeviation or for an ATC radar vector, the controllermust evaluate the air traffic picture in the affectedarea, and coordinate with other controllers (if ATCjurisdictional boundaries may be crossed) beforereplying to the request.

4. It should be remembered that the controller’sprimary function is to provide safe separationbetween aircraft. Any additional service, such asweather avoidance assistance, can only be providedto the extent that it does not derogate the primaryfunction. It’s also worth noting that the separationworkload is generally greater than normal whenweather disrupts the usual flow of traffic. ATC radarlimitations and frequency congestion may also be afactor in limiting the controller’s capability toprovide additional service.

5. It is very important, therefore, that the requestfor deviation or radar vector be forwarded to ATC asfar in advance as possible. Delay in submitting it maydelay or even preclude ATC approval or require thatadditional restrictions be placed on the clearance.Insofar as possible the following information should

be furnished to ATC when requesting clearance todetour around weather activity:

(a) Proposed point where detour will com-mence.

(b) Proposed route and extent of detour(direction and distance).

(c) Point where original route will beresumed.

(d) Flight conditions (IFR or VFR).

(e) Any further deviation that may becomenecessary as the flight progresses.

(f) Advise if the aircraft is equipped withfunctioning airborne radar.

6. To a large degree, the assistance that might berendered by ATC will depend upon the weatherinformation available to controllers. Due to theextremely transitory nature of severe weathersituations, the controller’s weather information maybe of only limited value if based on weather observedon radar only. Frequent updates by pilots givingspecific information as to the area affected, altitudes,intensity and nature of the severe weather can be ofconsiderable value. Such reports are relayed by radioor phone to other pilots and controllers and alsoreceive widespread teletypewriter dissemination.

7. Obtaining IFR clearance or an ATC radarvector to circumnavigate severe weather can often beaccommodated more readily in the en route areasaway from terminals because there is usually lesscongestion and, therefore, offer greater freedom ofaction. In terminal areas, the problem is more acutebecause of traffic density, ATC coordinationrequirements, complex departure and arrival routes,adjacent airports, etc. As a consequence, controllersare less likely to be able to accommodate all requestsfor weather detours in a terminal area or be in aposition to volunteer such routing to the pilot.Nevertheless, pilots should not hesitate to advisecontrollers of any observed severe weather andshould specifically advise controllers if they desirecircumnavigation of observed weather.

c. Procedures for Weather Deviations andOther Contingencies in Oceanic ControlledAirspace.

1. When the pilot initiates communications withATC, rapid response may be obtained by stating“WEATHER DEVIATION REQUIRED” to indicate

AIM 8/15/19


priority is desired on the frequency and for ATCresponse.

2. The pilot still retains the option of initiatingthe communications using the urgency call “PAN−PAN” 3 times to alert all listening parties of a specialhandling condition which will receive ATC priorityfor issuance of a clearance or assistance.

3. ATC will:

(a) Approve the deviation.

(b) Provide vertical separation and thenapprove the deviation; or

(c) If ATC is unable to establish verticalseparation, ATC must advise the pilot that standardseparation cannot be applied; provide essential trafficinformation for all affected aircraft, to the extentpracticable; and if possible, suggest a course ofaction. ATC may suggest that the pilot climb ordescend to a contingency altitude (1,000 feet above orbelow that assigned if operating above FL 290;500 feet above or below that assigned if operating ator below FL 290).

PHRASEOLOGY−STANDARD SEPARATION NOT AVAILABLE, DEVIATEAT PILOT’S DISCRETION; SUGGEST CLIMB (ordescent) TO (appropriate altitude); TRAFFIC (positionand altitude); REPORT DEVIATION COMPLETE.

4. The pilot will follow the ATC advisoryaltitude when approximately 10 NM from track aswell as execute the procedures detailed in para-graph 7−1−14c5.

5. If contact cannot be established or revisedATC clearance or advisory is not available anddeviation from track is required, the pilot must takethe following actions:

(a) If possible, deviate away from anorganized track or route system.

(b) Broadcast aircraft position and intentionson the frequency in use, as well as on frequency121.5 MHz at suitable intervals stating: flightidentification (operator call sign), flight level, trackcode or ATS route designator, and extent of deviationexpected.

(c) Watch for conflicting traffic both visuallyand by reference to TCAS (if equipped).

(d) Turn on aircraft exterior lights.

(e) Deviations of less than 10 NM shouldREMAIN at ASSIGNED altitude. Otherwise, whenthe aircraft is approximately 10 NM from track,initiate an altitude change based on the followingcriteria:

TBL 7−1−5

Route Centerline/Track

Deviations>10 NM Altitude Change

EAST(000�−179�

magnetic)

LEFT

RIGHT

DESCEND 300 ft

CLIMB 300 ft

WEST(180�−359�magnetic)

LEFT

RIGHT

CLIMB 300 ft

DESCEND 300 ft

Pilot Memory Slogan: “East right up, West right down.”

(f) When returning to track, be at assignedflight level when the aircraft is within approximately10 NM of centerline.

(g) If contact was not established prior todeviating, continue to attempt to contact ATC toobtain a clearance. If contact was established,continue to keep ATC advised of intentions andobtain essential traffic information.

7−1−15. Runway Visual Range (RVR)

There are currently two configurations of RVR in theNAS commonly identified as Taskers and NewGeneration RVR. The Taskers are the existingconfiguration which uses transmissometer technolo-gy. The New Generation RVRs were deployed inNovember 1994 and use forward scatter technology.The New Generation RVRs are currently beingdeployed in the NAS to replace the existing Taskers.

a. RVR values are measured by transmissometersmounted on 14−foot towers along the runway. A fullRVR system consists of:

1. Transmissometer projector and related items.

2. Transmissometer receiver (detector) andrelated items.

3. Analog

4. recorder.

5. Signal data converter and related items.

6. Remote digital or remote display program-mer.

AIM8/15/19


b. The transmissometer projector and receiver aremounted on towers 250 feet apart. A known intensityof light is emitted from the projector and is measuredby the receiver. Any obscuring matter such as rain,snow, dust, fog, haze or smoke reduces the lightintensity arriving at the receiver. The resultantintensity measurement is then converted to an RVRvalue by the signal data converter. These values aredisplayed by readout equipment in the associated airtraffic facility and updated approximately once everyminute for controller issuance to pilots.

c. The signal data converter receives informationon the high intensity runway edge light setting in use(step 3, 4, or 5); transmission values from thetransmissometer and the sensing of day or nightconditions. From the three data sources, the systemwill compute appropriate RVR values.

d. An RVR transmissometer established on a250 foot baseline provides digital readouts to aminimum of 600 feet, which are displayed in 200 footincrements to 3,000 feet and in 500 foot incrementsfrom 3,000 feet to a maximum value of 6,000 feet.

e. RVR values for Category IIIa operations extenddown to 700 feet RVR; however, only 600 and800 feet are reportable RVR increments. The800 RVR reportable value covers a range of 701 feetto 900 feet and is therefore a valid minimumindication of Category IIIa operations.

f. Approach categories with the correspondingminimum RVR values. (See TBL 7−1−6.)

TBL 7−1−6

Approach Category/Minimum RVR Table

Category Visibility (RVR)

Nonprecision 2,400 feet

Category I 1,800 feet*

Category II 1,000 feet

Category IIIa 700 feet

Category IIIb 150 feet

Category IIIc 0 feet

* 1,400 feet with special equipment and authorization

g. Ten minute maximum and minimum RVRvalues for the designated RVR runway are reported inthe body of the aviation weather report when theprevailing visibility is less than one mile and/or theRVR is 6,000 feet or less. ATCTs report RVR when

the prevailing visibility is 1 mile or less and/or theRVR is 6,000 feet or less.

h. Details on the requirements for the operationaluse of RVR are contained in FAA AC 97−1, RunwayVisual Range (RVR). Pilots are responsible forcompliance with minimums prescribed for their classof operations in the appropriate CFRs and/oroperations specifications.

i. RVR values are also measured by forwardscatter meters mounted on 14−foot frangiblefiberglass poles. A full RVR system consists of:

1. Forward scatter meter with a transmitter,receiver and associated items.

2. A runway light intensity monitor (RLIM).

3. An ambient light sensor (ALS).

4. A data processor unit (DPU).

5. Controller display (CD).

j. The forward scatter meter is mounted on a14−foot frangible pole. Infrared light is emitted fromthe transmitter and received by the receiver. Anyobscuring matter such as rain, snow, dust, fog, hazeor smoke increases the amount of scattered lightreaching the receiver. The resulting measurementalong with inputs from the runway light intensitymonitor and the ambient light sensor are forwarded tothe DPU which calculates the proper RVR value. TheRVR values are displayed locally and remotely oncontroller displays.

k. The runway light intensity monitors both therunway edge and centerline light step settings (steps 1through 5). Centerline light step settings are used forCAT IIIb operations. Edge Light step settings areused for CAT I, II, and IIIa operations.

l. New Generation RVRs can measure and displayRVR values down to the lowest limits ofCategory IIIb operations (150 feet RVR). RVRvalues are displayed in 100 feet increments and arereported as follows:

1. 100−feet increments for products below800 feet.

2. 200−feet increments for products between800 feet and 3,000 feet.

3. 500−feet increments for products between3,000 feet and 6,500 feet.

4. 25−meter increments for products below150 meters.

AIM 8/15/19


5. 50−meter increments for products between150 meters and 800 meters.

6. 100−meter increments for products between800 meters and 1,200 meters.

7. 200−meter increments for products between1,200 meters and 2,000 meters.

7−1−16. Reporting of Cloud Heights

a. Ceiling, by definition in the CFRs and as usedin aviation weather reports and forecasts, is the heightabove ground (or water) level of the lowest layer ofclouds or obscuring phenomenon that is reported as“broken,” “overcast,” or “obscuration,” e.g., anaerodrome forecast (TAF) which reads “BKN030”refers to height above ground level. An area forecastwhich reads “BKN030” indicates that the height isabove mean sea level.

REFERENCE−AIM, Paragraph 7−1−30 , Key to Aerodrome Forecast (TAF) and AviationRoutine Weather Report (METAR), defines “broken,” “overcast,” and“obscuration.”

b. Pilots usually report height values above MSL,since they determine heights by the altimeter. This istaken in account when disseminating and otherwiseapplying information received from pilots. (“Ceil-ing” heights are always above ground level.) Inreports disseminated as PIREPs, height referencesare given the same as received from pilots, that is,above MSL.

c. In area forecasts or inflight advisories, ceilingsare denoted by the contraction “CIG” when used withsky cover symbols as in “LWRG TO CIG OVC005,”or the contraction “AGL” after, the forecast cloudheight value. When the cloud base is given in heightabove MSL, it is so indicated by the contraction“MSL” or “ASL” following the height value. Theheights of clouds tops, freezing level, icing, andturbulence are always given in heights above ASL orMSL.

7−1−17. Reporting Prevailing Visibility

a. Surface (horizontal) visibility is reported inMETAR reports in terms of statute miles andincrements thereof; e.g., 1/16, 1/8, 3/

16, 1/4, 5/16, 3/

8, 1/2,

5/8, 3/4, 7/8, 1, 1 1/8, etc. (Visibility reported by an

unaugmented automated site is reported differentlythan in a manual report, i.e., ASOS/AWOS: 0, 1/16, 1/

8,1/4, 1/

2, 3/4, 1, 1 1/4, 1 1/2, 1 3/

4, 2, 2 1/2, 3, 4, 5, etc., AWOS:

M1/4, 1/4, 1/2, 3/4, 1, 1 1/4, 1 1/

2, 1 3/4, 2, 2 1/

2, 3, 4, 5, etc.)Visibility is determined through the ability to see andidentify preselected and prominent objects at aknown distance from the usual point of observation.Visibilities which are determined to be less than7 miles, identify the obscuring atmospheric condi-tion; e.g., fog, haze, smoke, etc., or combinationsthereof.

b. Prevailing visibility is the greatest visibilityequaled or exceeded throughout at least one half ofthe horizon circle, not necessarily contiguous.Segments of the horizon circle which may have asignificantly different visibility may be reported inthe remarks section of the weather report; i.e., thesoutheastern quadrant of the horizon circle may bedetermined to be 2 miles in mist while the remainingquadrants are determined to be 3 miles in mist.

c. When the prevailing visibility at the usual pointof observation, or at the tower level, is less than4 miles, certificated tower personnel will takevisibility observations in addition to those taken at theusual point of observation. The lower of these twovalues will be used as the prevailing visibility foraircraft operations.

7−1−18. Estimating Intensity of Rain andIce Pellets

a. Rain

1. Light. From scattered drops that, regardlessof duration, do not completely wet an exposed surfaceup to a condition where individual drops are easilyseen.

2. Moderate. Individual drops are not clearlyidentifiable; spray is observable just above pave-ments and other hard surfaces.

3. Heavy. Rain seemingly falls in sheets;individual drops are not identifiable; heavy spray toheight of several inches is observed over hardsurfaces.

b. Ice Pellets

1. Light. Scattered pellets that do not com-pletely cover an exposed surface regardless ofduration. Visibility is not affected.

2. Moderate. Slow accumulation on ground.Visibility reduced by ice pellets to less than 7 statutemiles.

AIM8/15/19


3. Heavy. Rapid accumulation on ground.Visibility reduced by ice pellets to less than 3 statutemiles.

7−1−19. Estimating Intensity of Snow orDrizzle (Based on Visibility)

a. Light. Visibility more than 1/2 statute mile.

b. Moderate. Visibility from more than1/4 statute mile to 1/2 statute mile.

c. Heavy. Visibility 1/4 statute mile or less.

7−1−20. Pilot Weather Reports (PIREPs)

a. FAA air traffic facilities are required to solicitPIREPs when the following conditions are reportedor forecast: ceilings at or below 5,000 feet; visibilityat or below 5 miles (surface or aloft); thunderstormsand related phenomena; icing of light degree orgreater; turbulence of moderate degree or greater;wind shear and reported or forecast volcanic ashclouds.

b. Pilots are urged to cooperate and promptlyvolunteer reports of these conditions and otheratmospheric data such as: cloud bases, tops andlayers; flight visibility; precipitation; visibilityrestrictions such as haze, smoke and dust; wind ataltitude; and temperature aloft.

c. PIREPs should be given to the ground facilitywith which communications are established; i.e.,FSS, ARTCC, or terminal ATC. One of the primaryduties of the Inflight position is to serve as acollection point for the exchange of PIREPs with enroute aircraft.

d. If pilots are not able to make PIREPs by radio,reporting upon landing of the inflight conditionsencountered to the nearest FSS or Weather ForecastOffice will be helpful. Some of the uses made of thereports are:

1. The ATCT uses the reports to expedite theflow of air traffic in the vicinity of the field and forhazardous weather avoidance procedures.

2. The FSS uses the reports to brief other pilots,to provide inflight advisories, and weather avoidanceinformation to en route aircraft.

3. The ARTCC uses the reports to expedite theflow of en route traffic, to determine most favorable

altitudes, and to issue hazardous weather informationwithin the center’s area.

4. The NWS uses the reports to verify or amendconditions contained in aviation forecast andadvisories. In some cases, pilot reports of hazardousconditions are the triggering mechanism for theissuance of advisories. They also use the reports forpilot weather briefings.

5. The NWS, other government organizations,the military, and private industry groups use PIREPsfor research activities in the study of meteorologicalphenomena.

6. All air traffic facilities and the NWS forwardthe reports received from pilots into the weatherdistribution system to assure the information is madeavailable to all pilots and other interested parties.

e. The FAA, NWS, and other organizations thatenter PIREPs into the weather reporting system usethe format listed in TBL 7−1−7. Items 1 through 6 areincluded in all transmitted PIREPs along with one ormore of items 7 through 13. Although the PIREPshould be as complete and concise as possible, pilotsshould not be overly concerned with strict format orphraseology. The important thing is that theinformation is relayed so other pilots may benefitfrom your observation. If a portion of the report needsclarification, the ground station will request theinformation. Completed PIREPs will be transmittedto weather circuits as in the following examples:

EXAMPLE−1. KCMH UA /OV APE 230010/TM 1516/FL085/TPBE20/SK BKN065/WX FV03SM HZ FU/TA 20/TB LGT

NOTE−1. One zero miles southwest of Appleton VOR; time1516 UTC; altitude eight thousand five hundred; aircrafttype BE200; bases of the broken cloud layer is six thousandfive hundred; flight visibility 3 miles with haze and smoke;air temperature 20 degrees Celsius; light turbulence.

EXAMPLE−2. KCRW UV /OV KBKW 360015−KCRW/TM1815/FL120//TP BE99/SK IMC/WX RA/TA M08 /WV290030/TB LGT−MDT/IC LGT RIME/RM MDT MXDICG DURC KROA NWBND FL080−100 1750Z

NOTE−2. From 15 miles north of Beckley VOR to Charles-ton VOR; time 1815 UTC; altitude 12,000 feet; typeaircraft, BE−99; in clouds; rain; temperature minus8 Celsius; wind 290 degrees magnetic at 30 knots; light tomoderate turbulence; light rime icing during climb

AIM 8/15/19


northwestbound from Roanoke, VA, between 8,000 and10,000 feet at 1750 UTC.

f. For more detailed information on PIREPS, users

can refer to the current version of AC 00−45, AviationWeather Services.

TBL 7−1−7

PIREP Element Code Chart

PIREP ELEMENT PIREP CODE CONTENTS

1. 3−letter station identifier XXX Nearest weather reporting location to the reported phenomenon

2. Report type UA or UUA Routine or Urgent PIREP

3. Location /OV In relation to a VOR

4. Time /TM Coordinated Universal Time

5. Altitude /FL Essential for turbulence and icing reports

6. Type Aircraft /TP Essential for turbulence and icing reports

7. Sky cover /SK Cloud height and coverage (sky clear, few, scattered, broken, orovercast)

8. Weather /WX Flight visibility, precipitation, restrictions to visibility, etc.

9. Temperature /TA Degrees Celsius

10. Wind /WV Direction in degrees magnetic north and speed in knots

11. Turbulence /TB See AIM paragraph 7−1−23

12. Icing /IC See AIM paragraph 7−1−21

13. Remarks /RM For reporting elements not included or to clarify previouslyreported items

7−1−21. PIREPs Relating to Airframe Icing

a. The effects of ice on aircraft are cumulative-thrust is reduced, drag increases, lift lessens, andweight increases. The results are an increase in stallspeed and a deterioration of aircraft performance. Inextreme cases, 2 to 3 inches of ice can form on theleading edge of the airfoil in less than 5 minutes. Ittakes but 1/2 inch of ice to reduce the lifting power ofsome aircraft by 50 percent and increases thefrictional drag by an equal percentage.

b. A pilot can expect icing when flying in visibleprecipitation, such as rain or cloud droplets, and thetemperature is between +02 and −10 degrees Celsius.When icing is detected, a pilot should do one of twothings, particularly if the aircraft is not equipped withdeicing equipment; get out of the area ofprecipitation; or go to an altitude where thetemperature is above freezing. This “warmer”altitude may not always be a lower altitude. Properpreflight action includes obtaining information on thefreezing level and the above freezing levels inprecipitation areas. Report icing to ATC, and ifoperating IFR, request new routing or altitude if icingwill be a hazard. Be sure to give the type of aircraft to

ATC when reporting icing. The following describeshow to report icing conditions.

1. Trace. Ice becomes perceptible. Rate ofaccumulation slightly greater than sublimation.Deicing/anti-icing equipment is not utilized unlessencountered for an extended period of time (over1 hour).

2. Light. The rate of accumulation may createa problem if flight is prolonged in this environment(over 1 hour). Occasional use of deicing/anti-icingequipment removes/prevents accumulation. It doesnot present a problem if the deicing/anti-icingequipment is used.

3. Moderate. The rate of accumulation is suchthat even short encounters become potentiallyhazardous and use of deicing/anti-icing equipment orflight diversion is necessary.

4. Severe. The rate of accumulation is such thatice protection systems fail to remove the accumula-tion of ice, or ice accumulates in locations notnormally prone to icing, such as areas aft of protectedsurfaces and any other areas identified by the

AIM8/15/19


manufacturer. Immediate exit from the condition isnecessary.

NOTE−Severe icing is aircraft dependent, as are the othercategories of icing intensity. Severe icing may occur at anyaccumulation rate.

EXAMPLE−Pilot report: give aircraft identification, location,time (UTC), intensity of type, altitude/FL, aircrafttype, indicated air speed (IAS), and outside air tempera-ture (OAT).

NOTE−

1. Rime ice. Rough, milky, opaque ice formed by theinstantaneous freezing of small supercooled waterdroplets.

2. Clear ice. A glossy, clear, or translucent ice formed bythe relatively slow freezing of large supercooled waterdroplets.

3. The OAT should be requested by the FSS or ATC if notincluded in the PIREP.

7−1−22. Definitions of Inflight Icing Terms

See TBL 7−1−8, Icing Types, and TBL 7−1−9, IcingConditions.

TBL 7−1−8

Icing Types

Clear Ice See Glaze Ice.

Glaze Ice Ice, sometimes clear and smooth, but usually containing some air pockets, which results in alumpy translucent appearance. Glaze ice results from supercooled drops/droplets striking asurface but not freezing rapidly on contact. Glaze ice is denser, harder, and sometimes moretransparent than rime ice. Factors, which favor glaze formation, are those that favor slowdissipation of the heat of fusion (i.e., slight supercooling and rapid accretion). With largeraccretions, the ice shape typically includes “horns” protruding from unprotected leading edgesurfaces. It is the ice shape, rather than the clarity or color of the ice, which is most likely tobe accurately assessed from the cockpit. The terms “clear” and “glaze” have been used foressentially the same type of ice accretion, although some reserve “clear” for thinner accretionswhich lack horns and conform to the airfoil.

Intercycle Ice Ice which accumulates on a protected surface between actuation cycles of a deicing system.

Known or Observed orDetected Ice Accretion

Actual ice observed visually to be on the aircraft by the flight crew or identified by on−boardsensors.

Mixed Ice Simultaneous appearance or a combination of rime and glaze ice characteristics. Since theclarity, color, and shape of the ice will be a mixture of rime and glaze characteristics, accurateidentification of mixed ice from the cockpit may be difficult.

Residual Ice Ice which remains on a protected surface immediately after the actuation of a deicing system.

Rime Ice A rough, milky, opaque ice formed by the rapid freezing of supercooled drops/droplets afterthey strike the aircraft. The rapid freezing results in air being trapped, giving the ice its opaqueappearance and making it porous and brittle. Rime ice typically accretes along the stagnationline of an airfoil and is more regular in shape and conformal to the airfoil than glaze ice. It isthe ice shape, rather than the clarity or color of the ice, which is most likely to be accuratelyassessed from the cockpit.

Runback Ice Ice which forms from the freezing or refreezing of water leaving protected surfaces andrunning back to unprotected surfaces.

Note−Ice types are difficult for the pilot to discern and have uncertain effects on an airplane in flight. Ice type definitions willbe included in the AIM for use in the “Remarks” section of the PIREP and for use in forecasting.

AIM 8/15/19


TBL 7−1−9

Icing Conditions

Appendix C Icing Conditions Appendix C (14 CFR, Part 25 and 29) is the certification icing condition standardfor approving ice protection provisions on aircraft. The conditions are specified interms of altitude, temperature, liquid water content (LWC), representative dropletsize (mean effective drop diameter [MED]), and cloud horizontal extent.

Forecast Icing Conditions Environmental conditions expected by a National Weather Service or anFAA−approved weather provider to be conducive to the formation of inflight icingon aircraft.

Freezing Drizzle (FZDZ) Drizzle is precipitation at ground level or aloft in the form of liquid water dropswhich have diameters less than 0.5 mm and greater than 0.05 mm. Freezing drizzleis drizzle that exists at air temperatures less than 0�C (supercooled), remains inliquid form, and freezes upon contact with objects on the surface or airborne.

Freezing Precipitation Freezing precipitation is freezing rain or freezing drizzle falling through or outsideof visible cloud.

Freezing Rain (FZRA) Rain is precipitation at ground level or aloft in the form of liquid water drops whichhave diameters greater than 0.5 mm. Freezing rain is rain that exists at airtemperatures less than 0�C (supercooled), remains in liquid form, and freezes uponcontact with objects on the ground or in the air.

Icing in Cloud Icing occurring within visible cloud. Cloud droplets (diameter < 0.05 mm) will bepresent; freezing drizzle and/or freezing rain may or may not be present.

Icing in Precipitation Icing occurring from an encounter with freezing precipitation, that is, supercooleddrops with diameters exceeding 0.05 mm, within or outside of visible cloud.

Known Icing Conditions Atmospheric conditions in which the formation of ice is observed or detected inflight.Note−Because of the variability in space and time of atmospheric conditions, the existenceof a report of observed icing does not assure the presence or intensity of icingconditions at a later time, nor can a report of no icing assure the absence of icingconditions at a later time.

Potential Icing Conditions Atmospheric icing conditions that are typically defined by airframe manufacturersrelative to temperature and visible moisture that may result in aircraft ice accretionon the ground or in flight. The potential icing conditions are typically defined in theAirplane Flight Manual or in the Airplane Operation Manual.

Supercooled Drizzle Drops(SCDD)

Synonymous with freezing drizzle aloft.

Supercooled Drops or /Droplets Water drops/droplets which remain unfrozen at temperatures below 0 �C.Supercooled drops are found in clouds, freezing drizzle, and freezing rain in theatmosphere. These drops may impinge and freeze after contact on aircraft surfaces.

Supercooled Large Drops (SLD) Liquid droplets with diameters greater than 0.05 mm at temperatures less than0�C, i.e., freezing rain or freezing drizzle.

AIM8/15/19


7−1−23. PIREPs Relating to Turbulence

a. When encountering turbulence, pilots areurgently requested to report such conditions to ATCas soon as practicable. PIREPs relating to turbulenceshould state:

1. Aircraft location.

2. Time of occurrence in UTC.

3. Turbulence intensity.

4. Whether the turbulence occurred in or nearclouds.

5. Aircraft altitude or flight level.

6. Type of aircraft.

7. Duration of turbulence.

EXAMPLE−1. Over Omaha, 1232Z, moderate turbulence in clouds atFlight Level three one zero, Boeing 707.

2. From five zero miles south of Albuquerque to three zeromiles north of Phoenix, 1250Z, occasional moderate chopat Flight Level three three zero, DC8.

b. Duration and classification of intensity shouldbe made using TBL 7−1−10.

TBL 7−1−10

Turbulence Reporting Criteria Table

Intensity Aircraft Reaction Reaction Inside Aircraft Reporting Term−Definition

Light Turbulence that momentarily causesslight, erratic changes in altitude and/orattitude (pitch, roll, yaw). Report asLight Turbulence; 1orTurbulence that causes slight, rapid andsomewhat rhythmic bumpiness withoutappreciable changes in altitude orattitude. Report as Light Chop.

Occupants may feel a slight strainagainst seat belts or shoulder straps.Unsecured objects may be displacedslightly. Food service may be con-ducted and little or no difficulty isencountered in walking.

Occasional−Less than 1/3 of the time.

Intermittent−1/3 to 2/3.

Continuous−More than 2/3.

Moderate Turbulence that is similar to LightTurbulence but of greater intensity.Changes in altitude and/or attitude occurbut the aircraft remains in positivecontrol at all times. It usually causesvariations in indicated airspeed. Reportas Moderate Turbulence; 1orTurbulence that is similar to Light Chopbut of greater intensity. It causes rapidbumps or jolts without appreciablechanges in aircraft altitude or attitude.Report as Moderate Chop.1

Occupants feel definite strains againstseat belts or shoulder straps. Unse-cured objects are dislodged. Foodservice and walking are difficult.

NOTE1. Pilots should report location(s),time (UTC), intensity, whether in ornear clouds, altitude, type of aircraftand, when applicable, duration ofturbulence.

2. Duration may be based on timebetween two locations or over a singlelocation. All locations should bereadily identifiable.

Severe Turbulence that causes large, abruptchanges in altitude and/or attitude. Itusually causes large variations inindicated airspeed. Aircraft may bemomentarily out of control. Report asSevere Turbulence. 1

Occupants are forced violently againstseat belts or shoulder straps. Unse-cured objects are tossed about. FoodService and walking are impossible.

EXAMPLES:a. Over Omaha. 1232Z, ModerateTurbulence, in cloud, FlightLevel 310, B707.

Extreme Turbulence in which the aircraft isviolently tossed about and is practicallyimpossible to control. It may causestructural damage. Report as ExtremeTurbulence. 1

b. From 50 miles south of Albuquer-que to 30 miles north of Phoenix,1210Z to 1250Z, occasional ModerateChop, Flight Level 330, DC8.

1 High level turbulence (normally above 15,000 feet ASL) not associated with cumuliform cloudiness, including thunderstorms,should be reported as CAT (clear air turbulence) preceded by the appropriate intensity, or light or moderate chop.

AIM 8/15/19


7−1−24. Wind Shear PIREPs

a. Because unexpected changes in wind speed anddirection can be hazardous to aircraft operations atlow altitudes on approach to and departing fromairports, pilots are urged to promptly volunteerreports to controllers of wind shear conditions theyencounter. An advance warning of this informationwill assist other pilots in avoiding or coping with awind shear on approach or departure.

b. When describing conditions, use of the terms“negative” or “positive” wind shear should beavoided. PIREPs of “negative wind shear on final,”intended to describe loss of airspeed and lift, havebeen interpreted to mean that no wind shear wasencountered. The recommended method for windshear reporting is to state the loss or gain of airspeedand the altitudes at which it was encountered.

EXAMPLE−1. Denver Tower, Cessna 1234 encountered wind shear,loss of 20 knots at 400.

2. Tulsa Tower, American 721 encountered wind shear onfinal, gained 25 knots between 600 and 400 feet followedby loss of 40 knots between 400 feet and surface.

1. Pilots who are not able to report wind shear inthese specific terms are encouraged to make reportsin terms of the effect upon their aircraft.

EXAMPLE−Miami Tower, Gulfstream 403 Charlie encountered anabrupt wind shear at 800 feet on final, max thrust required.

2. Pilots using Inertial Navigation Systems(INSs) should report the wind and altitude both aboveand below the shear level.

7−1−25. Clear Air Turbulence (CAT) PIREPs

CAT has become a very serious operational factor toflight operations at all levels and especially to jet

traffic flying in excess of 15,000 feet. The bestavailable information on this phenomenon mustcome from pilots via the PIREP reporting procedures.All pilots encountering CAT conditions are urgentlyrequested to report time, location, and intensity (light,moderate, severe, or extreme) of the element to theFAA facility with which they are maintaining radiocontact. If time and conditions permit, elementsshould be reported according to the standards forother PIREPs and position reports.

REFERENCE−AIM, Paragraph 7−1−23 , PIREPs Relating to Turbulence

7−1−26. Microbursts

a. Relatively recent meteorological studies haveconfirmed the existence of microburst phenomenon.Microbursts are small scale intense downdraftswhich, on reaching the surface, spread outward in alldirections from the downdraft center. This causes thepresence of both vertical and horizontal wind shearsthat can be extremely hazardous to all types andcategories of aircraft, especially at low altitudes. Dueto their small size, short life span, and the fact thatthey can occur over areas without surface precipita-tion, microbursts are not easily detectable usingconventional weather radar or wind shear alertsystems.

b. Parent clouds producing microburst activitycan be any of the low or middle layer convectivecloud types. Note, however, that microburstscommonly occur within the heavy rain portion ofthunderstorms, and in much weaker, benignappearing convective cells that have little or noprecipitation reaching the ground.

AIM8/15/19


FIG 7−1−12

Evolution of a Microburst

T-5 MinT-5 Min T-2 MinT-2 Min TT T + 5 MinT + 5 Min T + 10 MinT + 10 Min

HE

IGH

T (

feet)

HE

IGH

T (

feet)

10,00010,000

5,0005,000

WIND SPEEDWIND SPEED

10-20 knots10-20 knots

> 20 knots> 20 knots

SCALE (miles)SCALE (miles)

00 1 2 3

Vertical cross section of the evolution of a microburst wind field. T is the time of initial divergence atthe surface. The shading refers to the vector wind speeds. Figure adapted from Wilson et al., 1984,Microburst Wind Structure and Evaluation of Doppler Radar for Wind Shear Detection, DOT/FAAReport No. DOT/FAA/PM-84/29, National Technical Information Service, Springfield, VA 37 pp.

c. The life cycle of a microburst as it descends ina convective rain shaft is seen in FIG 7−1−12. Animportant consideration for pilots is the fact that themicroburst intensifies for about 5 minutes after itstrikes the ground.

d. Characteristics of microbursts include:

1. Size. The microburst downdraft is typicallyless than 1 mile in diameter as it descends from thecloud base to about 1,000−3,000 feet above theground. In the transition zone near the ground, thedowndraft changes to a horizontal outflow that canextend to approximately 2 1/2 miles in diameter.

2. Intensity. The downdrafts can be as strongas 6,000 feet per minute. Horizontal winds near thesurface can be as strong as 45 knots resulting in a90 knot shear (headwind to tailwind change for atraversing aircraft) across the microburst. Thesestrong horizontal winds occur within a few hundredfeet of the ground.

3. Visual Signs. Microbursts can be foundalmost anywhere that there is convective activity.They may be embedded in heavy rain associated witha thunderstorm or in light rain in benign appearingvirga. When there is little or no precipitation at thesurface accompanying the microburst, a ring ofblowing dust may be the only visual clue of itsexistence.

4. Duration. An individual microburst willseldom last longer than 15 minutes from the time itstrikes the ground until dissipation. The horizontalwinds continue to increase during the first 5 minuteswith the maximum intensity winds lasting approxi-mately 2−4 minutes. Sometimes microbursts areconcentrated into a line structure, and under theseconditions, activity may continue for as long as anhour. Once microburst activity starts, multiplemicrobursts in the same general area are notuncommon and should be expected.

AIM 8/15/19


FIG 7−1−13

Microburst Encounter During Takeoff

A microburst encounter during takeoff. The airplane first encounters a headwind and experiences increasingperformance (1), this is followed in short succession by a decreasing headwind component (2), a downdraft(3), and finally a strong tailwind (4), where 2 through 5 all result in decreasing performance of the airplane.Position (5) represents an extreme situation just prior to impact. Figure courtesy of Walter Frost, FWGAssociates, Inc., Tullahoma, Tennessee.

e. Microburst wind shear may create a severehazard for aircraft within 1,000 feet of the ground,particularly during the approach to landing andlanding and take-off phases. The impact of amicroburst on aircraft which have the unfortunate

experience of penetrating one is characterized inFIG 7−1−13. The aircraft may encounter a headwind(performance increasing) followed by a downdraftand tailwind (both performance decreasing), possiblyresulting in terrain impact.

AIM8/15/19


FIG 7−1−14

NAS Wind Shear Product Systems

(33)(39)(36)(9)

f. Detection of Microbursts, Wind Shear andGust Fronts.

1. FAA’s Integrated Wind Shear DetectionPlan.

(a) The FAA currently employs an integratedplan for wind shear detection that will significantlyimprove both the safety and capacity of the majorityof the airports currently served by the air carriers.This plan integrates several programs, such as theIntegrated Terminal Weather System (ITWS),Terminal Doppler Weather Radar (TDWR), WeatherSystem Processor (WSP), and Low Level Wind ShearAlert Systems (LLWAS) into a single strategic

concept that significantly improves the aviationweather information in the terminal area. (SeeFIG 7−1−14.)

(b) The wind shear/microburst informationand warnings are displayed on the ribbon displayterminals (RBDT) located in the tower cabs. They areidentical (and standardized) in the LLWAS, TDWRand WSP systems, and so designed that the controllerdoes not need to interpret the data, but simply read thedisplayed information to the pilot. The RBDTs areconstantly monitored by the controller to ensure therapid and timely dissemination of any hazardousevent(s) to the pilot.

AIM 8/15/19


FIG 7−1−15

LLWAS Siting Criteria

(c) The early detection of a wind shear/mi-cro−burst event, and the subsequent warning(s)issued to an aircraft on approach or departure, willalert the pilot/crew to the potential of, and to beprepared for, a situation that could become verydangerous! Without these warnings, the aircraft mayNOT be able to climb out of, or safely transition, theevent, resulting in a catastrophe. The air carriers,working with the FAA, have developed specializedtraining programs using their simulators to train andprepare their pilots on the demanding aircraftprocedures required to escape these very dangerouswind shear and/or microburst encounters.

2. Low Level Wind Shear Alert System(LLWAS).

(a) The LLWAS provides wind data andsoftware processes to detect the presence ofhazardous wind shear and microbursts in the vicinityof an airport. Wind sensors, mounted on polessometimes as high as 150 feet, are (ideally) located2,000 − 3,500 feet, but not more than 5,000 feet, fromthe centerline of the runway. (See FIG 7−1−15.)

AIM8/15/19


FIG 7−1−16

Warning Boxes

(b) LLWAS was fielded in 1988 at 110 air-ports across the nation. Many of these systems havebeen replaced by new TDWR and WSP technology.Eventually all LLWAS systems will be phased out;however, 39 airports will be upgraded to theLLWAS−NE (Network Expansion) system, whichemploys the very latest software and sensortechnology. The new LLWAS−NE systems will notonly provide the controller with wind shear warningsand alerts, including wind shear/microburst detectionat the airport wind sensor location, but will alsoprovide the location of the hazards relative to theairport runway(s). It will also have the flexibility andcapability to grow with the airport as new runways arebuilt. As many as 32 sensors, strategically locatedaround the airport and in relationship to its runwayconfiguration, can be accommodated by theLLWAS−NE network.

3. Terminal Doppler Weather Radar (TD-WR).

(a) TDWRs are being deployed at 45 loca-tions across the U.S. Optimum locations for TDWRsare 8 to 12 miles off of the airport proper, anddesigned to look at the airspace around and over theairport to detect microbursts, gust fronts, wind shifts

and precipitation intensities. TDWR products advisethe controller of wind shear and microburst eventsimpacting all runways and the areas 1/2 mile on eitherside of the extended centerline of the runways out to3 miles on final approach and 2 miles out ondeparture.(FIG 7−1−16 is a theoretical view of the warningboxes, including the runway, that the software uses indetermining the location(s) of wind shear ormicrobursts). These warnings are displayed (asdepicted in the examples in subparagraph 5) on theRBDT.

(b) It is very important to understand whatTDWR does NOT DO:

(1) It DOES NOT warn of wind shearoutside of the alert boxes (on the arrival and departureends of the runways);

(2) It DOES NOT detect wind shear that isNOT a microburst or a gust front;

(3) It DOES NOT detect gusty or crosswind conditions; and

(4) It DOES NOT detect turbulence.

However, research and development is continuing onthese systems. Future improvements may includesuch areas as storm motion (movement), improved

AIM 8/15/19


gust front detection, storm growth and decay,microburst prediction, and turbulence detection.

(c) TDWR also provides a geographicalsituation display (GSD) for supervisors and trafficmanagement specialists for planning purposes. TheGSD displays (in color) 6 levels of weather(precipitation), gust fronts and predicted stormmovement(s). This data is used by the towersupervisor(s), traffic management specialists andcontrollers to plan for runway changes andarrival/departure route changes in order to bothreduce aircraft delays and increase airport capacity.

4. Weather System Processor (WSP).

(a) The WSP provides the controller, supervi-sor, traffic management specialist, and ultimately thepilot, with the same products as the terminal dopplerweather radar (TDWR) at a fraction of the cost of aTDWR. This is accomplished by utilizing newtechnologies to access the weather channel capabili-ties of the existing ASR−9 radar located on or near theairport, thus eliminating the requirements for aseparate radar location, land acquisition, supportfacilities and the associated communication landlinesand expenses.

(b) The WSP utilizes the same RBDT displayas the TDWR and LLWAS, and, just like TDWR, alsohas a GSD for planning purposes by supervisors,traffic management specialists and controllers. TheWSP GSD emulates the TDWR display, i.e., it alsodepicts 6 levels of precipitation, gust fronts andpredicted storm movement, and like the TDWR GSD,is used to plan for runway changes and arrival/depar-ture route changes in order to reduce aircraft delaysand to increase airport capacity.

(c) This system is currently under develop-ment and is operating in a developmental test statusat the Albuquerque, New Mexico, airport. Whenfielded, the WSP is expected to be installed at

34 airports across the nation, substantially increasingthe safety of the American flying public.

5. Operational aspects of LLWAS, TDWRand WSP.

To demonstrate how this data is used by both thecontroller and the pilot, 3 ribbon display examplesand their explanations are presented:

(a) MICROBURST ALERTS

EXAMPLE−This is what the controller sees on his/her ribbon displayin the tower cab.

27A MBA 35K− 2MF 250 20

NOTE−(See FIG 7−1−17 to see how the TDWR/WSP determinesthe microburst location).

This is what the controller will say when issuing thealert.

PHRASEOLOGY−RUNWAY 27 ARRIVAL, MICROBURST ALERT, 35 KTLOSS 2 MILE FINAL, THRESHOLD WIND 250 AT 20.

In plain language, the controller is telling the pilotthat on approach to runway 27, there is a microburstalert on the approach lane to the runway, and toanticipate or expect a 35 knot loss of airspeed atapproximately 2 miles out on final approach (whereit will first encounter the phenomena). With thatinformation, the aircrew is forewarned, and should beprepared to apply wind shear/microburst escapeprocedures should they decide to continue theapproach. Additionally, the surface winds at theairport for landing runway 27 are reported as250 degrees at 20 knots.

NOTE−Threshold wind is at pilot’s request or as deemedappropriate by the controller.

REFERENCE−FAA Order JO 7110.65, Paragraph 3−1−8b2(a), Air Traffic Control, LowLevel Wind Shear/Microburst Advisories

AIM8/15/19


FIG 7−1−17

Microburst Alert

(b) WIND SHEAR ALERTSEXAMPLE−This is what the controller sees on his/her ribbon displayin the tower cab.

27A WSA 20K− 3MF 200 15

NOTE−(See FIG 7−1−18 to see how the TDWR/WSP determinesthe wind shear location).

This is what the controller will say when issuing thealert.PHRASEOLOGY−RUNWAY 27 ARRIVAL, WIND SHEAR ALERT, 20 KTLOSS 3 MILE FINAL, THRESHOLD WIND 200 AT 15.

In plain language, the controller is advising theaircraft arriving on runway 27 that at about 3 milesout they can expect to encounter a wind shearcondition that will decrease their airspeed by 20 knotsand possibly encounter turbulence. Additionally, theairport surface winds for landing runway 27 arereported as 200 degrees at 15 knots.

NOTE−Threshold wind is at pilot’s request or as deemedappropriate by the controller.

REFERENCE−FAA Order JO 7110.65, Air Traffic Control, Low Level WindShear/Microburst Advisories, Paragraph 3−1−8b2(a).

AIM 8/15/19


FIG 7−1−18

Weak Microburst Alert

AIM8/15/19


FIG 7−1−19

Gust Front Alert

(c) MULTIPLE WIND SHEAR ALERTS

EXAMPLE−This is what the controller sees on his/her ribbon displayin the tower cab.

27A WSA 20K+ RWY 250 20

27D WSA 20K+ RWY 250 20

NOTE−(See FIG 7−1−19 to see how the TDWR/WSP determinesthe gust front/wind shear location.)

This is what the controller will say when issuing thealert.

PHRASEOLOGY−MULTIPLE WIND SHEAR ALERTS. RUNWAY 27ARRIVAL, WIND SHEAR ALERT, 20 KT GAIN ONRUNWAY; RUNWAY 27 DEPARTURE, WIND SHEARALERT, 20 KT GAIN ON RUNWAY, WIND 250 AT 20.

EXAMPLE−In this example, the controller is advising arriving anddeparting aircraft that they could encounter a wind shearcondition right on the runway due to a gust front(significant change of wind direction) with the possibilityof a 20 knot gain in airspeed associated with the gust front.Additionally, the airport surface winds (for the runway inuse) are reported as 250 degrees at 20 knots.

REFERENCE−FAA Order 7110.65, Air Traffic Control, Low Level WindShear/Microburst Advisories, Paragraph 3−1−8b2(d).

AIM 8/15/19


6. The Terminal Weather Information forPilots System (TWIP).

(a) With the increase in the quantity andquality of terminal weather information availablethrough TDWR, the next step is to provide thisinformation directly to pilots rather than relying onvoice communications from ATC. The NationalAirspace System has long been in need of a means ofdelivering terminal weather information to thecockpit more efficiently in terms of both speed andaccuracy to enhance pilot awareness of weatherhazards and reduce air traffic controller workload.With the TWIP capability, terminal weatherinformation, both alphanumerically and graphically,is now available directly to the cockpit at 43 airportsin the U.S. NAS. (See FIG 7−1−20.)

FIG 7−1−20

TWIP Image of Convective Weather at MCO International

(b) TWIP products are generated usingweather data from the TDWR or the IntegratedTerminal Weather System (ITWS) testbed. TWIPproducts are generated and stored in the form of textand character graphic messages. Software has beendeveloped to allow TDWR or ITWS to format thedata and send the TWIP products to a databaseresident at Aeronautical Radio, Inc. (ARINC). Theseproducts can then be accessed by pilots using theARINC Aircraft Communications Addressing andReporting System (ACARS) data link services.Airline dispatchers can also access this database andsend messages to specific aircraft whenever windshear activity begins or ends at an airport.

(c) TWIP products include descriptions andcharacter graphics of microburst alerts, wind shearalerts, significant precipitation, convective activity

within 30 NM surrounding the terminal area, andexpected weather that will impact airport operations.During inclement weather, i.e., whenever a predeter-mined level of precipitation or wind shear is detectedwithin 15 miles of the terminal area, TWIP productsare updated once each minute for text messages andonce every five minutes for character graphicmessages. During good weather (below the predeter-mined precipitation or wind shear parameters) eachmessage is updated every 10 minutes. These productsare intended to improve the situational awareness ofthe pilot/flight crew, and to aid in flight planning priorto arriving or departing the terminal area. It isimportant to understand that, in the context of TWIP,the predetermined levels for inclement versus goodweather has nothing to do with the criteria forVFR/MVFR/IFR/LIFR; it only deals with precipita-tion, wind shears and microbursts.

TBL 7−1−11

TWIP−Equipped Airports

Airport Identifier

Andrews AFB, MD KADW

Hartsfield−Jackson Atlanta Intl Airport KATL

Nashville Intl Airport KBNA

Logan Intl Airport KBOS

Baltimore/Washington Intl Airport KBWI

Hopkins Intl Airport KCLE

Charlotte/Douglas Intl Airport KCLT

Port Columbus Intl Airport KCMH

Cincinnati/Northern Kentucky Intl Airport KCVG

Dallas Love Field Airport KDAL

James M. Cox Intl Airport KDAY

Ronald Reagan Washington National Air-port

KDCA

Denver Intl Airport KDEN

Dallas−Fort Worth Intl Airport KDFW

Detroit Metro Wayne County Airport KDTW

Newark Liberty Intl Airport KEWR

Fort Lauderdale−Hollywood Intl Airport KFLL

William P. Hobby Airport KHOU

Washington Dulles Intl Airport KIAD

George Bush Intercontinental Airport KIAH

Wichita Mid−Continent Airport KICT

Indianapolis Intl Airport KIND

AIM8/15/19


Airport Identifier

John F. Kennedy Intl Airport KJFK

LaGuardia Airport KLGA

Kansas City Intl Airport KMCI

Orlando Intl Airport KMCO

Midway Intl Airport KMDW

Memphis Intl Airport KMEM

Miami Intl Airport KMIA

General Mitchell Intl Airport KMKE

Minneapolis St. Paul Intl Airport KMSP

Louis Armstrong New Orleans Intl Air-port

KMSY

Will Rogers World Airport KOKC

O’Hare Intl Airport KORD

Palm Beach Intl Airport KPBI

Philadelphia Intl Airport KPHL

Pittsburgh Intl Airport KPIT

Raleigh−Durham Intl Airport KRDU

Louisville Intl Airport KSDF

Salt Lake City Intl Airport KSLC

Lambert−St. Louis Intl Airport KSTL

Tampa Intl Airport KTPA

Tulsa Intl Airport KTUL

7−1−27. PIREPs Relating to Volcanic AshActivity

a. Volcanic eruptions which send ash into theupper atmosphere occur somewhere around the worldseveral times each year. Flying into a volcanic ashcloud can be extremely dangerous. At least twoB747s have lost all power in all four engines aftersuch an encounter. Regardless of the type aircraft,some damage is almost certain to ensue after anencounter with a volcanic ash cloud. Additionally,studies have shown that volcanic eruptions are theonly significant source of large quantities of sulphurdioxide (SO2) gas at jet-cruising altitudes. Therefore,the detection and subsequent reporting of SO2 is ofsignificant importance. Although SO2 is colorless, itspresence in the atmosphere should be suspected whena sulphur-like or rotten egg odor is present throughoutthe cabin.

b. While some volcanoes in the U.S. aremonitored, many in remote areas are not. Theseunmonitored volcanoes may erupt without priorwarning to the aviation community. A pilot observinga volcanic eruption who has not had previousnotification of it may be the only witness to theeruption. Pilots are strongly encouraged to transmit aPIREP regarding volcanic eruptions and anyobserved volcanic ash clouds or detection of sulphurdioxide (SO2) gas associated with volcanic activity.

c. Pilots should submit PIREPs regarding volcanicactivity using the Volcanic Activity Reporting (VAR)form as illustrated in Appendix 2. If a VAR form isnot immediately available, relay enough informationto identify the position and type of volcanic activity.

d. Pilots should verbally transmit the data requiredin items 1 through 8 of the VAR as soon as possible.The data required in items 9 through 16 of the VARshould be relayed after landing if possible.

7−1−28. Thunderstorms

a. Turbulence, hail, rain, snow, lightning, sus-tained updrafts and downdrafts, icing conditions−allare present in thunderstorms. While there is someevidence that maximum turbulence exists at themiddle level of a thunderstorm, recent studies showlittle variation of turbulence intensity with altitude.

b. There is no useful correlation between theexternal visual appearance of thunderstorms and theseverity or amount of turbulence or hail within them.The visible thunderstorm cloud is only a portion of aturbulent system whose updrafts and downdraftsoften extend far beyond the visible storm cloud.Severe turbulence can be expected up to 20 milesfrom severe thunderstorms. This distance decreasesto about 10 miles in less severe storms.

c. Weather radar, airborne or ground based, willnormally reflect the areas of moderate to heavyprecipitation (radar does not detect turbulence). Thefrequency and severity of turbulence generallyincreases with the radar reflectivity which is closelyassociated with the areas of highest liquid watercontent of the storm. NO FLIGHT PATH THROUGHAN AREA OF STRONG OR VERY STRONGRADAR ECHOES SEPARATED BY 20−30 MILESOR LESS MAY BE CONSIDERED FREE OFSEVERE TURBULENCE.

d. Turbulence beneath a thunderstorm should notbe minimized. This is especially true when the

AIM 8/15/19


relative humidity is low in any layer between thesurface and 15,000 feet. Then the lower altitudes maybe characterized by strong out flowing winds andsevere turbulence.

e. The probability of lightning strikes occurring toaircraft is greatest when operating at altitudes wheretemperatures are between minus 5 degrees Celsiusand plus 5 degrees Celsius. Lightning can strikeaircraft flying in the clear in the vicinity of athunderstorm.

f. METAR reports do not include a descriptor forsevere thunderstorms. However, by understandingsevere thunderstorm criteria, i.e., 50 knot winds or3/4 inch hail, the information is available in the reportto know that one is occurring.

g. Current weather radar systems are able toobjectively determine precipitation intensity. Theseprecipitation intensity areas are described as “light,”“moderate,” “heavy,” and “extreme.”

REFERENCE−Pilot/Controller Glossary− Precipitation Radar Weather Descriptions

EXAMPLE−1. Alert provided by an ATC facility to an aircraft:(aircraft identification) EXTREME precipitation betweenten o’clock and two o’clock, one five miles. Precipitationarea is two five miles in diameter.

2. Alert provided by an FSS:(aircraft identification) EXTREME precipitation two zeromiles west of Atlanta V−O−R, two five miles wide, movingeast at two zero knots, tops flight level three niner zero.

7−1−29. Thunderstorm Flying

a. Thunderstorm Avoidance. Never regard anythunderstorm lightly, even when radar echoes are oflight intensity. Avoiding thunderstorms is the bestpolicy. Following are some Do’s and Don’ts ofthunderstorm avoidance:

1. Don’t land or takeoff in the face of anapproaching thunderstorm. A sudden gust front oflow level turbulence could cause loss of control.

2. Don’t attempt to fly under a thunderstormeven if you can see through to the other side.Turbulence and wind shear under the storm could behazardous.

3. Don’t attempt to fly under the anvil of athunderstorm. There is a potential for severe andextreme clear air turbulence.

4. Don’t fly without airborne radar into a cloudmass containing scattered embedded thunderstorms.Scattered thunderstorms not embedded usually canbe visually circumnavigated.

5. Don’t trust the visual appearance to be areliable indicator of the turbulence inside athunderstorm.

6. Don’t assume that ATC will offer radarnavigation guidance or deviations around thunder-storms.

7. Don’t use data-linked weather next genera-tion weather radar (NEXRAD) mosaic imagery as thesole means for negotiating a path through athunderstorm area (tactical maneuvering).

8. Do remember that the data-linked NEXRADmosaic imagery shows where the weather was, notwhere the weather is. The weather conditions may be15 to 20 minutes older than the age indicated on thedisplay.

9. Do listen to chatter on the ATC frequency forPilot Weather Reports (PIREP) and other aircraftrequesting to deviate or divert.

10. Do ask ATC for radar navigation guidanceor to approve deviations around thunderstorms, ifneeded.

11. Do use data-linked weather NEXRADmosaic imagery (for example, Flight InformationService-Broadcast (FIS-B)) for route selection toavoid thunderstorms entirely (strategic maneuver-ing).

12. Do advise ATC, when switched to anothercontroller, that you are deviating for thunderstormsbefore accepting to rejoin the original route.

13. Do ensure that after an authorized weatherdeviation, before accepting to rejoin the originalroute, that the route of flight is clear of thunderstorms.

14. Do avoid by at least 20 miles anythunderstorm identified as severe or giving an intenseradar echo. This is especially true under the anvil ofa large cumulonimbus.

15. Do circumnavigate the entire area if the areahas 6/10 thunderstorm coverage.

16. Do remember that vivid and frequentlightning indicates the probability of a severethunderstorm.

AIM8/15/19


17. Do regard as extremely hazardous anythunderstorm with tops 35,000 feet or higher whetherthe top is visually sighted or determined by radar.

18. Do give a PIREP for the flight conditions.

19. Do divert and wait out the thunderstorms onthe ground if unable to navigate around an area ofthunderstorms.

20. Do contact Flight Service for assistance inavoiding thunderstorms. Flight Service specialistshave NEXRAD mosaic radar imagery and NEXRADsingle site radar with unique features such as base andcomposite reflectivity, echo tops, and VAD windprofiles.

b. If you cannot avoid penetrating a thunderstorm,following are some Do’s before entering the storm:

1. Tighten your safety belt, put on your shoulderharness (if installed), if and secure all loose objects.

2. Plan and hold the course to take the aircraftthrough the storm in a minimum time.

3. To avoid the most critical icing, establish apenetration altitude below the freezing level or abovethe level of -15ºC.

4. Verify that pitot heat is on and turn oncarburetor heat or jet engine anti-ice. Icing can berapid at any altitude and cause almost instantaneouspower failure and/or loss of airspeed indication.

5. Establish power settings for turbulencepenetration airspeed recommended in the aircraftmanual.

6. Turn up cockpit lights to highest intensity tolessen temporary blindness from lightning.

7. If using automatic pilot, disengage AltitudeHold Mode and Speed Hold Mode. The automaticaltitude and speed controls will increase maneuversof the aircraft thus increasing structural stress.

8. If using airborne radar, tilt the antenna up anddown occasionally. This will permit the detection ofother thunderstorm activity at altitudes other than theone being flown.

c. Following are some Do’s and Don’ts during thethunderstorm penetration:

1. Do keep your eyes on your instruments.Looking outside the cockpit can increase danger oftemporary blindness from lightning.

2. Don’t change power settings; maintainsettings for the recommended turbulence penetrationairspeed.

3. Do maintain constant attitude. Allow thealtitude and airspeed to fluctuate.

4. Don’t turn back once you are in thethunderstorm. A straight course through the stormmost likely will get the aircraft out of the hazardsmost quickly. In addition, turning maneuvers increasestress on the aircraft.

AIM 8/15/19


7−1−30. Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR)

FIG 7−1−21

Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR) (Front)

Key to Aerodrome Forecast (TAF) and Aviation

Routine Weather Report (METAR) (Front)

TAF KPIT 091730Z 0918/1024 15005KT 5SM HZ FEW020 WS010/31022KT

FM091930 30015G25KT 3SM SHRA OVC015

TEMPO 0920/0922 1/2SM +TSRA OVC008CB

FM100100 27008KT 5SM SHRA BKN020 OVC040

PROB30 1004/1007 1SM -RA BR

FM101015 18005KT 6SM -SHRA OVC020

BECMG 1013/1015 P6SM NSW SKC

NOTE: Users are cautioned to confirm DATE and TIME of the TAF. For example FM100000 is

0000Z on the 10th. Do not confuse with 1000Z!METAR KPIT 091955Z COR 22015G25KT 3/4SM R28L/2600FT TSRA OVC010CB 18/16 A2992 RMK

SLP045 T01820159

Forecast Explanation ReportTAF Message type: TAF-routine or TAF AMD-amended forecast, METAR-

hourly, SPECI-special or TESTM-non-commissioned ASOS report

METAR

KPIT ICAO location indicator KPIT091730Z Issuance time: ALL times in UTC “Z”, 2-digit date, 4-digit time 091955Z0918/1024 Valid period, either 24 hours or 30 hours. The first two digits of EACH

four digit number indicate the date of the valid period, the final two di

gits indicate the time (valid from 18Z on the 9th to 24Z on the 10th).In U.S. METAR: CORrected ob; or AUTOmated ob for automated re

port with no human intervention; omitted when observer logs on.

COR

15005KT Wind: 3 digit true-north direction, nearest 10 degrees (or VaRiaBle);

next 2-3 digits for speed and unit, KT (KMH or MPS); as needed, Gust

and maximum speed; 00000KT for calm; for METAR, if direction varies

60 degrees or more, Variability appended, e.g., 180V260

22015G25KT

5SM Prevailing visibility; in U.S., Statute Miles & fractions; above 6 miles in

TAF Plus6SM. (Or, 4-digit minimum visibility in meters and as re

quired, lowest value with direction)

¾SM

Runway Visual Range: R; 2-digit runway designator Left, Center, or

Right as needed; “/”, Minus or Plus in U.S., 4-digit value, FeeT in U.S.,

(usually meters elsewhere); 4-digit value Variability 4-digit value (and

tendency Down, Up or No change)

R28L/2600FT

HZ Significant present, forecast and recent weather: see table (on back) TSRAFEW020 Cloud amount, height and type: Sky Clear 0/8, FEW >0/8-2/8, ScaTtered

3/8-4/8, BroKeN 5/8-7/8, OverCast 8/8; 3-digit height in hundreds of ft;

Towering Cumulus or CumulonimBus in METAR; in TAF, only CB.

Vertical Visibility for obscured sky and height “VV004”. More than 1

layer may be reported or forecast. In automated METAR reports only,

CleaR for “clear below 12,000 feet”

OVC 010CB

Temperature: degrees Celsius; first 2 digits, temperature “/” last 2 digits,

dew-point temperature; Minus for below zero, e.g., M06

18/16

Altimeter setting: indicator and 4 digits; in U.S., A-inches and hun

dredths; (Q-hectoPascals, e.g., Q1013)

A2992

WS010/31022KT In U.S. TAF, non-convective low-level (≤ 2,000 ft) Wind Shear; 3-digit

height (hundreds of ft); “/”; 3-digit wind direction and 2-3 digit wind

speed above the indicated height, and unit, KT

AIM8/15/19


FIG 7−1−22

Key to Aerodrome Forecast (TAF) and Aviation Routine Weather Report (METAR) (Back)

Key to Aerodrome Forecast (TAF) and Aviation

Routine Weather Report (METAR) (Back)

In METAR, ReMarK indicator & remarks. For example: Sea- Level

Pressure in hectoPascals & tenths, as shown: 1004.5 hPa; Temp/dew-

point in tenths °C, as shown: temp. 18.2°C, dew-point 15.9°C

RMK SLP045

T01820159

FM091930 FroM: changes are expected at: 2-digit date, 2-digit hour, and 2-digit

minute beginning time: indicates significant change. Each FM starts on a

new line, indented 5 spacesTEMPO

0920/0922

TEMPOrary: changes expected for <1 hour and in total, < half of the

period between the 2-digit date and 2-digit hour beginning, and 2-digit

date and 2-digit hour ending timePROB30

1004/1007

PROBability and 2-digit percent (30 or 40): probable condition in the

period between the 2-digit date & 2-digit hour beginning time, and the

2-digit date and 2-digit hour ending timeBECMG

1013/1015

BECoMinG: change expected in the period between the 2-digit date and

2-digit hour beginning time, and the 2-digit date and 2-digit hour ending

time

Table of Significant Present, Forecast and Recent Weather - Grouped in categories and

used in the order listed below; or as needed in TAF, No Significant Weather.QualifiersIntensity or Proximity“-” = Light No sign = Moderate “+” = Heavy“VC” = Vicinity, but not at aerodrome. In the US METAR, 5 to 10 SM from the point of observation. In the US

TAF, 5 to 10 SM from the center of the runway complex. Elsewhere, within 8000m.

DescriptorBC – Patches BL – Blowing DR – Drifting FZ – FreezingMI – Shallow PR – Partial SH – Showers TS – Thunderstorm

Weather PhenomenaPrecipitationDZ – Drizzle GR – Hail GS – Small Hail/Snow PelletsIC – Ice Crystals PL – Ice Pellets RA – Rain SG – Snow GrainsSN – Snow UP – Unknown Precipitation in automated observations

ObscurationBR – Mist (≥5/8SM) DU – Widespread Dust FG – Fog (<5/8SM) FU – SmokeHZ – Haze PY – Spray SA – Sand VA – Volcanic Ash

OtherDS – Dust Storm FC – Funnel Cloud +FC – Tornado or WaterspoutPO – Well developed dust or sand whirls SQ – Squall SS – Sandstorm

- Explanations in parentheses “()” indicate different worldwide practices.- Ceiling is not specified; defined as the lowest broken or overcast layer, or the vertical visibility.- NWS TAFs exclude BECMG groups and temperature forecasts, NWS TAFS do not use PROB in the first 9

hours of a TAF; NWS METARs exclude trend forecasts. US Military TAFs include Turbulence and Icing groups.

AIM 8/15/19


7−1−31. International Civil AviationOrganization (ICAO) Weather Formats

The U.S. uses the ICAO world standard for aviationweather reporting and forecasting. The WorldMeteorological Organization’s (WMO) publicationNo. 782 “Aerodrome Reports and Forecasts”contains the base METAR and TAF code as adoptedby the WMO member countries.

a. Although the METAR code is adoptedworldwide, each country is allowed to makemodifications or exceptions to the code for use intheir particular country, e.g., the U.S. will continue touse statute miles for visibility, feet for RVR values,knots for wind speed, and inches of mercury foraltimetry. However, temperature and dew point willbe reported in degrees Celsius. The U.S reportsprevailing visibility rather than lowest sectorvisibility. The elements in the body of a METARreport are separated with a space. The only exceptionsare RVR, temperature, and dew point which areseparated with a solidus (/). When an element doesnot occur, or cannot be observed, the preceding spaceand that element are omitted from that particularreport. A METAR report contains the followingsequence of elements in the following order:

1. Type of report.

2. ICAO Station Identifier.

3. Date and time of report.

4. Modifier (as required).

5. Wind.

6. Visibility.

7. Runway Visual Range (RVR).

8. Weather phenomena.

9. Sky conditions.

10. Temperature/dew point group.

11. Altimeter.

12. Remarks (RMK).

b. The following paragraphs describe the ele-ments in a METAR report.

1. Type of report. There are two types ofreport:

(a) Aviation Routine Weather Report(METAR); and

(b) Nonroutine (Special) Aviation WeatherReport (SPECI).

The type of report (METAR or SPECI) will alwaysappear as the lead element of the report.

2. ICAO Station Identifier. The METARcode uses ICAO 4−letter station identifiers. In thecontiguous 48 States, the 3−letter domestic stationidentifier is prefixed with a “K;” i.e., the domesticidentifier for Seattle is SEA while the ICAO identifieris KSEA. Elsewhere, the first two letters of the ICAOidentifier indicate what region of the world andcountry (or state) the station is in. For Alaska, allstation identifiers start with “PA;” for Hawaii, allstation identifiers start with “PH.” Canadian stationidentifiers start with “CU,” “CW,” “CY,” and “CZ.”Mexican station identifiers start with “MM.” Theidentifier for the western Caribbean is “M” followedby the individual country’s letter; i.e., Cuba is “MU;”Dominican Republic “MD;” the Bahamas “MY.” Theidentifier for the eastern Caribbean is “T” followedby the individual country’s letter; i.e., Puerto Rico is“TJ.” For a complete worldwide listing see ICAODocument 7910, Location Indicators.

3. Date and Time of Report. The date andtime the observation is taken are transmitted as asix−digit date/time group appended with Z to denoteCoordinated Universal Time (UTC). The first twodigits are the date followed with two digits for hourand two digits for minutes.

EXAMPLE−172345Z (the 17th day of the month at 2345Z)

4. Modifier (As Required). “AUTO” identi-fies a METAR/SPECI report as an automated weatherreport with no human intervention. If “AUTO” isshown in the body of the report, the type of sensorequipment used at the station will be encoded in theremarks section of the report. The absence of“AUTO” indicates that a report was made manuallyby an observer or that an automated report had humanaugmentation/backup. The modifier “COR” indi-cates a corrected report that is sent out to replace anearlier report with an error.

NOTE−There are two types of automated stations, AO1 forautomated weather reporting stations without a precipita-tion discriminator, and AO2 for automated stations with aprecipitation discriminator. (A precipitation discriminatorcan determine the difference between liquid andfrozen/freezing precipitation). This information appears inthe remarks section of an automated report.

AIM8/15/19


5. Wind. The wind is reported as a five digitgroup (six digits if speed is over 99 knots). The firstthree digits are the direction the wind is blowingfrom, in tens of degrees referenced to true north, or“VRB” if the direction is variable. The next two digitsis the wind speed in knots, or if over 99 knots, the nextthree digits. If the wind is gusty, it is reported as a “G”after the speed followed by the highest gust reported.The abbreviation “KT” is appended to denote the useof knots for wind speed.

EXAMPLE−13008KT − wind from 130 degrees at 8 knots08032G45KT − wind from 080 degrees at 32 knots withgusts to 45 knotsVRB04KT − wind variable in direction at 4 knots00000KT − wind calm210103G130KT − wind from 210 degrees at 103 knots withgusts to 130 knotsIf the wind direction is variable by 60 degrees or more andthe speed is greater than 6 knots, a variable groupconsisting of the extremes of the wind direction separatedby a “v” will follow the prevailing wind group.32012G22KT 280V350

(a) Peak Wind. Whenever the peak windexceeds 25 knots “PK WND” will be included inRemarks, e.g., PK WND 28045/1955 “Peak wind twoeight zero at four five occurred at one niner five five.”If the hour can be inferred from the report time, onlythe minutes will be appended, e.g., PK WND34050/38 “Peak wind three four zero at five zerooccurred at three eight past the hour.”

(b) Wind shift. Whenever a wind shiftoccurs, “WSHFT” will be included in remarksfollowed by the time the wind shift began, e.g.,WSHFT 30 FROPA “Wind shift at three zero due tofrontal passage.”

6. Visibility. Prevailing visibility is reported instatute miles with “SM” appended to it.

EXAMPLE−7SM − seven statute miles15SM − fifteen statute miles1/2SM − one−half statute mile

(a) Tower/surface visibility. If either visi-bility (tower or surface) is below four statute miles,

the lesser of the two will be reported in the body of thereport; the greater will be reported in remarks.

(b) Automated visibility. ASOS/AWOSvisibility stations will show visibility 10 or greaterthan 10 miles as “10SM.” AWOS visibility stationswill show visibility less than 1/4 statute mile as“M1/4SM” and visibility 10 or greater than 10 milesas “10SM.”

NOTE−Automated sites that are augmented by human observer tomeet service level requirements can report 0, 1/16 SM, and1/8 SM visibility increments.

(c) Variable visibility. Variable visibility isshown in remarks (when rapid increase or decreaseby 1/2 statute mile or more and the average prevailingvisibility is less than three miles) e.g., VIS 1V2“visibility variable between one and two.”

(d) Sector visibility. Sector visibility isshown in remarks when it differs from the prevailingvisibility, and either the prevailing or sector visibilityis less than three miles.

EXAMPLE−VIS N2 − visibility north two

7. Runway Visual Range (When Reported).“R” identifies the group followed by the runwayheading (and parallel runway designator, if needed)“/” and the visual range in feet (meters in othercountries) followed with “FT” (feet is not spoken).

(a) Variability Values. When RVR varies(by more than on reportable value), the lowest andhighest values are shown with “V” between them.

(b) Maximum/Minimum Range. “P” indi-cates an observed RVR is above the maximum valuefor this system (spoken as “more than”). “M”indicates an observed RVR is below the minimumvalue which can be determined by the system (spokenas “less than”).

EXAMPLE−R32L/1200FT − runway three two left R−V−R one thousandtwo hundred.R27R/M1000V4000FT − runway two seven right R−V−Rvariable from less than one thousand to four thousand.

AIM 8/15/19


8. Weather Phenomena. The weather asreported in the METAR code represents a significantchange in the way weather is currently reported. InMETAR, weather is reported in the format:

Intensity/Proximity/Descriptor/Precipitation/Ob-struction to visibility/Other

NOTE−The “/” above and in the following descriptions (except asthe separator between the temperature and dew point) arefor separation purposes in this publication and do notappear in the actual METARs.

(a) Intensity applies only to the first type ofprecipitation reported. A “−” denotes light, no symboldenotes moderate, and a “+” denotes heavy.

(b) Proximity applies to and reported onlyfor weather occurring in the vicinity of the airport(between 5 and 10 miles of the point(s) ofobservation). It is denoted by the letters “VC.”(Intensity and “VC” will not appear together in theweather group).

(c) Descriptor. These eight descriptors ap-ply to the precipitation or obstructions to visibility:

TS thunderstorm. . . . . . . . . . .DR low drifting. . . . . . . . . . .SH showers. . . . . . . . . . .MI shallow. . . . . . . . . . .FZ freezing. . . . . . . . . . .BC patches. . . . . . . . . . .BL blowing. . . . . . . . . . .PR partial. . . . . . . . . . .

NOTE−Although “TS” and “SH” are used with precipitation andmay be preceded with an intensity symbol, the intensity stillapplies to the precipitation, not the descriptor.

(d) Precipitation. There are nine types ofprecipitation in the METAR code:

RA rain. . . . . . . . . .DZ drizzle. . . . . . . . . .SN snow. . . . . . . . . .GR hail (1/4” or greater). . . . . . . . . .GS small hail/snow pellets. . . . . . . . . .PL ice pellets. . . . . . . . . .SG snow grains. . . . . . . . . .IC ice crystals (diamond dust). . . . . . . . . . .UP unknown precipitation. . . . . . . . . .

(automated stations only)

(e) Obstructions to visibility. There areeight types of obscuration phenomena in the METARcode (obscurations are any phenomena in theatmosphere, other than precipitation, that reducehorizontal visibility):

FG fog (vsby less than 5/8 mile). . . . . . . . . .HZ haze. . . . . . . . . .FU smoke. . . . . . . . . .PY spray. . . . . . . . . .BR mist (vsby 5/8 − 6 miles). . . . . . . . . .SA sand. . . . . . . . . .DU dust. . . . . . . . . .VA volcanic ash. . . . . . . . . .

NOTE−Fog (FG) is observed or forecast only when the visibility isless than five−eighths of mile, otherwise mist (BR) isobserved or forecast.

(f) Other. There are five categories of otherweather phenomena which are reported when theyoccur:

SQ squall. . . . . . . . . . .SS sandstorm. . . . . . . . . . .DS duststorm. . . . . . . . . . .PO dust/sand whirls. . . . . . . . . .FC funnel cloud. . . . . . . . . . .+FC tornado/waterspout. . . . . . . . .

Examples:

TSRA thunderstorm with moderate. . . . . . . . . rain

+SN heavy snow. . . . . . . . . .−RA FG light rain and fog. . . . . . .BRHZ mist and haze . . . . . . . .

(visibility 5/8 mile or greater)FZDZ freezing drizzle. . . . . . . . .VCSH rain shower in the vicinity. . . . . . . .+SHRASNPL heavy rain showers, snow,. .

ice pellets (intensityindicator refers to the predominant rain)

9. Sky Condition. The sky condition asreported in METAR represents a significant changefrom the way sky condition is currently reported. InMETAR, sky condition is reported in the format:

Amount/Height/(Type) or Indefinite Ceiling/Height

AIM8/15/19


(a) Amount. The amount of sky cover isreported in eighths of sky cover, using thecontractions:

SKC clear (no clouds). . . . . . . . .FEW >0 to 2/8. . . . . . . .SCT scattered (3/8s to 4/8s of. . . . . . . . .

clouds)BKN broken (5/8s to 7/8s of clouds). . . . . . . . .OVC overcast (8/8s clouds). . . . . . . . .CB Cumulonimbus when present. . . . . . . . . .TCU Towering cumulus when. . . . . . . . .

present

NOTE−1. “SKC” will be reported at manual stations. “CLR” willbe used at automated stations when no clouds below12,000 feet are reported.

2. A ceiling layer is not designated in the METAR code.For aviation purposes, the ceiling is the lowest broken orovercast layer, or vertical visibility into an obscuration.Also there is no provision for reporting thin layers in theMETAR code. When clouds are thin, that layer must bereported as if it were opaque.

(b) Height. Cloud bases are reported withthree digits in hundreds of feet above ground level(AGL). (Clouds above 12,000 feet cannot be reportedby an automated station).

(c) (Type). If Towering Cumulus Clouds(TCU) or Cumulonimbus Clouds (CB) are present,they are reported after the height which representstheir base.

EXAMPLE−(Reported as) SCT025TCU BKN080 BKN250 (spoken as)“TWO THOUSAND FIVE HUNDRED SCATTEREDTOWERING CUMULUS, CEILING EIGHT THOUSANDBROKEN, TWO FIVE THOUSAND BROKEN.”(Reported as) SCT008 OVC012CB (spoken as) “EIGHTHUNDRED SCATTERED CEILING ONE THOUSANDTWO HUNDRED OVERCAST CUMULONIMBUSCLOUDS.”

(d) Vertical Visibility (indefinite ceilingheight). The height into an indefinite ceiling ispreceded by “VV” and followed by three digitsindicating the vertical visibility in hundreds of feet.This layer indicates total obscuration.

EXAMPLE−1/8 SM FG VV006 − visibility one eighth, fog, indefiniteceiling six hundred.

(e) Obscurations are reported when the skyis partially obscured by a ground−based phenomenaby indicating the amount of obscuration as FEW,SCT, BKN followed by three zeros (000). In remarks,the obscuring phenomenon precedes the amount ofobscuration and three zeros.

EXAMPLE−BKN000 (in body) “sky partially obscured”. . . . . . . .FU BKN000 (in remarks) “smoke obscuring five−. . .

to seven−eighths of thesky”

(f) When sky conditions include a layer aloft,other than clouds, such as smoke or haze the type ofphenomena, sky cover and height are shown inremarks.

EXAMPLE−BKN020 (in body) “ceiling two thousand. . . . . . . .

broken”RMK FU BKN020 “broken layer of smoke. . . . . . . .

aloft, based at two thousand”

(g) Variable ceiling. When a ceiling isbelow three thousand and is variable, the remark“CIG” will be shown followed with the lowest andhighest ceiling heights separated by a “V.”

EXAMPLE−CIG 005V010 “ceiling variable . . . . . . . . . . . .

between five hundred andone thousand”

(h) Second site sensor. When an automatedstation uses meteorological discontinuity sensors,remarks will be shown to identify site specific skyconditions which differ and are lower than conditionsreported in the body.

EXAMPLE−CIG 020 RY11 “ceiling two thousand at . . . . . . . . . . .

runway one one”

(i) Variable cloud layer. When a layer isvarying in sky cover, remarks will show thevariability range. If there is more than one cloudlayer, the variable layer will be identified byincluding the layer height.

EXAMPLE−SCT V BKN “scattered layer variable to. . . . . . . . . . . . .

broken”BKN025 V OVC “broken layer at . . . . . . . . .

two thousand five hundredvariable to overcast”

AIM 8/15/19


(j) Significant clouds. When significantclouds are observed, they are shown in remarks,along with the specified information as shown below:

(1) Cumulonimbus (CB), or Cumulonim-bus Mammatus (CBMAM), distance (if known),direction from the station, and direction ofmovement, if known. If the clouds are beyond10 miles from the airport, DSNT will indicatedistance.

EXAMPLE−CB W MOV E “cumulonimbus west moving. . . . . . .

east”CBMAM DSNT S “cumulonimbus mammatus . . . .

distant south”

(2) Towering Cumulus (TCU), location, (ifknown), or direction from the station.

EXAMPLE−TCU OHD “towering cumulus overhead”. . . . . . . . .TCU W “towering cumulus west”. . . . . . . . . . . .

(3) Altocumulus Castellanus (ACC), Stra-tocumulus Standing Lenticular (SCSL),Altocumulus Standing Lenticular (ACSL), Cirrocu-mulus Standing Lenticular (CCSL) or rotor clouds,describing the clouds (if needed) and the directionfrom the station.

EXAMPLE−ACC W “altocumulus castellanus west”. . . . . . . . . . . . .ACSL SW−S “standing lenticular . . . . . . . . .

altocumulus southwestthrough south”

APRNT ROTOR CLD S “apparent rotor cloud south”CCSL OVR MT E “standing lenticular . . . . .

cirrocumulus over the mountains east”

10. Temperature/Dew Point. Temperatureand dew point are reported in two, two-digit groupsin degrees Celsius, separated by a solidus (“/”).Temperatures below zero are prefixed with an “M.”If the temperature is available but the dew point ismissing, the temperature is shown followed by asolidus. If the temperature is missing, the group isomitted from the report.

EXAMPLE−15/08 “temperature one five, . . . . . . . . . . . . . .

dew point 8”00/M02 “temperature zero,. . . . . . . . . . . .

dew point minus 2”

M05/ “temperature minus five, . . . . . . . . . . . . . . .dew point missing”

11. Altimeter. Altimeter settings are reportedin a four-digit format in inches of mercury prefixedwith an “A” to denote the units of pressure.

EXAMPLE−A2995 − “Altimeter two niner niner five”

12. Remarks. Remarks will be included in allobservations, when appropriate. The contraction“RMK” denotes the start of the remarks section of aMETAR report.

Except for precipitation, phenomena located within5 statute miles of the point of observation will bereported as at the station. Phenomena between 5 and10 statute miles will be reported in the vicinity, “VC.”Precipitation not occurring at the point of observationbut within 10 statute miles is also reported as in thevicinity, “VC.” Phenomena beyond 10 statute mileswill be shown as distant, “DSNT.” Distances are instatute miles except for automated lightning remarkswhich are in nautical miles. Movement of clouds orweather will be indicated by the direction towardwhich the phenomena is moving.

(a) There are two categories of remarks:

(1) Automated, manual, and plain lan-guage.

(2) Additive and automated maintenancedata.

(b) Automated, Manual, and Plain Lan-guage. This group of remarks may be generatedfrom either manual or automated weather reportingstations and generally elaborate on parametersreported in the body of the report. (Plain languageremarks are only provided by manual stations).

(1) Volcanic eruptions.

(2) Tornado, Funnel Cloud, Waterspout.

(3) Station Type (AO1 or AO2).

(4) PK WND.

(5) WSHFT (FROPA).

(6) TWR VIS or SFC VIS.

(7) VRB VIS.

(8) Sector VIS.

(9) VIS @ 2nd Site.

AIM8/15/19


(10) Lightning. When lightning is observedat a manual location, the frequency and location isreported.

When cloud−to−ground lightning is detected by anautomated lightning detection system, such asALDARS:

[a] Within 5 nautical miles (NM) of theAirport Reference Point (ARP), it will be reported as“TS” in the body of the report with no remark;

[b] Between 5 and 10 NM of the ARP, itwill be reported as “VCTS” in the body of the reportwith no remark;

[c] Beyond 10 but less than 30 NM of theARP, it will be reported in remarks as “DSNT”followed by the direction from the ARP.

EXAMPLE−LTG DSNT W or LTG DSNT ALQDS

(11) Beginning/Ending of Precipitation/TSTMS.

(12) TSTM Location MVMT.

(13) Hailstone Size (GR).

(14) Virga.

(15) VRB CIG (height).

(16) Obscuration.

(17) VRB Sky Condition.

(18) Significant Cloud Types.

(19) Ceiling Height 2nd Location.

(20) PRESFR PRESRR.

(21) Sea−Level Pressure.

(22) ACFT Mishap (not transmitted).

(23) NOSPECI.

(24) SNINCR.

(25) Other SIG Info.

(c) Additive and Automated MaintenanceData.

(1) Hourly Precipitation.

(2) 3− and 6−Hour Precipitation Amount.

(3) 24−Hour Precipitation.

(4) Snow Depth on Ground.

(5) Water Equivalent of Snow.

(6) Cloud Type.

(7) Duration of Sunshine.

(8) Hourly Temperature/Dew Point(Tenths).

(9) 6−Hour Maximum Temperature.

(10) 6−Hour Minimum Temperature.

(11) 24−Hour Maximum/MinimumTemperature.

(12) Pressure Tendency.

(13) Sensor Status.PWINOFZRANOTSNORVRNOPNOVISNO

Examples of METAR reports and explanation:

METAR KBNA 281250Z 33018KT 290V3601/2SM R31/2700FT SN BLSN FG VV008 00/M03A2991 RMK RAE42SNB42

METAR aviation routine weather . . . . . .report

KBNA Nashville, TN. . . . . . . .281250Z date 28th, time 1250 UTC. . . . . .(no modifier) This is a manually generated. .

report, due to the absence of“AUTO” and “AO1 or AO2”in remarks

33018KT wind three three zero at one. . . . .eight

290V360 wind variable between. . . . . .two nine zero and three sixzero

1/2SM visibility one half. . . . . . . .R31/2700FT Runway three one RVR two. . .

thousand seven hundredSN moderate snow. . . . . . . . . . .BLSN FG visibility obscured by . . . . .

blowing snow and fogVV008 indefinite ceiling eight. . . . . . .

hundred00/M03 temperature zero, dew point. . . . . . .

minus threeA2991 altimeter two niner niner one. . . . . . . .RMK remarks. . . . . . . .

AIM 8/15/19


RAE42 rain ended at four two. . . . . . .SNB42 snow began at four two. . . . . . .

METAR KSFO 041453Z AUTO VRB02KT 3SMBR CLR 15/12 A3012 RMK AO2

METAR aviation routine weather . . . . . .report

KSFO San Francisco, CA. . . . . . . .041453Z date 4th, time 1453 UTC. . . . . .AUTO fully automated; no human. . . . . . .

interventionVRB02KT wind variable at two. . . .3SM visibility three. . . . . . . . .BR visibility obscured by mist. . . . . . . . . .CLR no clouds below one two. . . . . . . . .

thousand15/12 temperature one five, dew. . . . . . . . .

point one twoA3012 altimeter three zero one two. . . . . . . .RMK remarks. . . . . . . .AO2 this automated station has a. . . . . . . . .

weather discriminator (forprecipitation)

SPECI KCVG 152224Z 28024G36KT 3/4SM+TSRA BKN008 OVC020CB 28/23 A3000 RMKTSRAB24 TS W MOV E

SPECI (nonroutine) aviation special. . . . . . .weather report

KCVG Cincinnati, OH. . . . . . .152228Z date 15th, time 2228 UTC. . . . . .(no modifier) This is a manually generated. .

report due to the absence of“AUTO” and “AO1 or AO2”in remarks

28024G36KT wind two eight zero at . .two four gusts three six

3/4SM visibility three fourths. . . . . . . .+TSRA thunderstorms, heavy rain. . . . . . .BKN008 ceiling eight hundred brokenOVC020CB two thousand overcast. . .

cumulonimbus clouds28/23 temperature two eight, . . . . . . . . .

dew point two threeA3000 altimeter three zero zero zero. . . . . . . .RMK remarks . . . . . . . .TSRAB24 thunderstorm and rain began. . . . .

at two four

TS W MOV E thunderstorm west movingeast

c. Aerodrome Forecast (TAF). A concise state-ment of the expected meteorological conditions at anairport during a specified period. At most locations,TAFs have a 24 hour forecast period. However, TAFsfor some locations have a 30 hour forecast period.These forecast periods may be shorter in the case ofan amended TAF. TAFs use the same codes asMETAR weather reports. They are scheduled fourtimes daily for 24−hour periods beginning at 0000Z,0600Z, 1200Z, and 1800Z.

Forecast times in the TAF are depicted in two ways.The first is a 6−digit number to indicate a specificpoint in time, consisting of a two−digit date,two−digit hour, and two−digit minute (such asissuance time or FM). The second is a pair offour−digit numbers separated by a “/” to indicate abeginning and end for a period of time. In this case,each four−digit pair consists of a two−digit date anda two−digit hour.

TAFs are issued in the following format:

TYPE OF REPORT/ICAO STATION IDENTIFIER/DATE AND TIME OF ORIGIN/VALID PERIODDATE AND TIME/FORECAST METEOROLOGI-CAL CONDITIONS

NOTE−The “/” above and in the following descriptions are forseparation purposes in this publication and do not appearin the actual TAFs.

TAF KORD 051130Z 0512/0618 14008KT 5SM BRBKN030

TEMPO 0513/0516 1 1/2SM BRFM051600 16010KT P6SM SKCFM052300 20013G20KT 4SM SHRA OVC020PROB40 0600/0606 2SM TSRA OVC008CB BECMG 0606/0608 21015KT P6SM NSW

SCT040

TAF format observed in the above example:

TAF = type of report

KORD = ICAO station identifier

051130Z = date and time of origin (issuance time)

0512/0618 = valid period date and times

14008KT 5SM BR BKN030 = forecast meteorologi-cal conditions

Explanation of TAF elements:

AIM8/15/19


1. Type of Report. There are two types of TAFissuances, a routine forecast issuance (TAF) and anamended forecast (TAF AMD). An amended TAF isissued when the current TAF no longer adequatelydescribes the on-going weather or the forecaster feelsthe TAF is not representative of the current orexpected weather. Corrected (COR) or delayed(RTD) TAFs are identified only in the communica-tions header which precedes the actual forecasts.

2. ICAO Station Identifier. The TAF codeuses ICAO 4−letter location identifiers as describedin the METAR section.

3. Date and Time of Origin. This element isthe date and time the forecast is actually prepared.The format is a two−digit date and four−digit timefollowed, without a space, by the letter “Z.”

4. Valid Period Date and Time. The UTCvalid period of the forecast consists of two four−digitsets, separated by a “/”. The first four−digit set is atwo−digit date followed by the two−digit beginninghour, and the second four−digit set is a two−digit datefollowed by the two−digit ending hour. Althoughmost airports have a 24−hour TAF, a select number ofairports have a 30−hour TAF. In the case of anamended forecast, or a forecast which is corrected ordelayed, the valid period may be for less than 24hours. Where an airport or terminal operates on apart−time basis (less than 24 hours/day), the TAFsissued for those locations will have the abbreviatedstatement “AMD NOT SKED” added to the end ofthe forecasts. The time observations are scheduled toend and/or resume will be indicated by expanding theAMD NOT SKED statement. Expanded statementswill include:

(a) Observation ending time (AFT DDHH-mm; for example, AFT 120200)

(b) Scheduled observations resumption time(TIL DDHHmm; for example, TIL 171200Z) or

(c) Period of observation unavailability(DDHH/DDHH); for example, 2502/2512).

5. Forecast Meteorological Conditions. Thisis the body of the TAF. The basic format is:

WIND/VISIBILITY/WEATHER/SKY CONDI-TION/OPTIONAL DATA (WIND SHEAR)

The wind, visibility, and sky condition elements arealways included in the initial time group of theforecast. Weather is included only if significant to

aviation. If a significant, lasting change in any of theelements is expected during the valid period, a newtime period with the changes is included. It should benoted that with the exception of a “FM” group thenew time period will include only those elementswhich are expected to change, i.e., if a lowering of thevisibility is expected but the wind is expected toremain the same, the new time period reflecting thelower visibility would not include a forecast wind.The forecast wind would remain the same as in theprevious time period. Any temporary conditionsexpected during a specific time period are includedwith that time period. The following describes theelements in the above format.

(a) Wind. This five (or six) digit groupincludes the expected wind direction (first 3 digits)and speed (last 2 digits or 3 digits if 100 knots orgreater). The contraction “KT” follows to denote theunits of wind speed. Wind gusts are noted by the letter“G” appended to the wind speed followed by thehighest expected gust. A variable wind direction isnoted by “VRB” where the three digit directionusually appears. A calm wind (3 knots or less) isforecast as “00000KT.”

EXAMPLE−18010KT wind one eight zero at one zero (wind is. . . . .

blowing from 180).35012G20KT wind three five zero at one two gust two. .

zero.

(b) Visibility. The expected prevailing visi-bility up to and including 6 miles is forecast in statutemiles, including fractions of miles, followed by “SM”to note the units of measure. Expected visibilitiesgreater than 6 miles are forecast as P6SM (plussix statute miles).

EXAMPLE−1/2SM − visibility one−half4SM − visibility fourP6SM − visibility more than six

(c) Weather Phenomena. The expectedweather phenomena is coded in TAF reports using thesame format, qualifiers, and phenomena contractionsas METAR reports (except UP). Obscurations tovision will be forecast whenever the prevailingvisibility is forecast to be 6 statute miles or less. If nosignificant weather is expected to occur during aspecific time period in the forecast, the weatherphenomena group is omitted for that time period. If,after a time period in which significant weatherphenomena has been forecast, a change to a forecast

AIM 8/15/19


of no significant weather phenomena occurs, thecontraction NSW (No Significant Weather) willappear as the weather group in the new time period.(NSW is included only in TEMPO groups).

NOTE−It is very important that pilots understand that NSW onlyrefers to weather phenomena, i.e., rain, snow, drizzle, etc.Omitted conditions, such as sky conditions, visibility,winds, etc., are carried over from the previous time group.

(d) Sky Condition. TAF sky conditionforecasts use the METAR format described in theMETAR section. Cumulonimbus clouds (CB) are theonly cloud type forecast in TAFs. When clear skiesare forecast, the contraction “SKC” will always beused. The contraction “CLR” is never used in theTAF. When the sky is obscured due to asurface−based phenomenon, vertical visibility (VV)into the obscuration is forecast. The format forvertical visibility is “VV” followed by a three−digitheight in hundreds of feet.

NOTE−As in METAR, ceiling layers are not designated in the TAFcode. For aviation purposes, the ceiling is the lowestbroken or overcast layer or vertical visibility into acomplete obscuration.

SKC “sky clear”. . . . . . . . . . . . . .SCT005 BKN025CB “five hundred scattered,.

ceiling two thousandfive hundred broken cumulonimbus clouds”

VV008 “indefinite ceiling. . . . . . . . . . . .eight hundred”

(e) Optional Data (Wind Shear). Windshear is the forecast of nonconvective low level winds(up to 2,000 feet). The forecast includes the letters“WS” followed by the height of the wind shear, thewind direction and wind speed at the indicated heightand the ending letters “KT” (knots). Height is givenin hundreds of feet (AGL) up to and including2,000 feet. Wind shear is encoded with thecontraction “WS,” followed by a three−digit height,slant character “/,” and winds at the height indicatedin the same format as surface winds. The wind shearelement is omitted if not expected to occur.

WS010/18040KT − “LOW LEVEL WIND SHEARAT ONE THOUSAND, WIND ONE EIGHT ZEROAT FOUR ZERO”

d. Probability Forecast. The probability orchance of thunderstorms or other precipitation events

occurring, along with associated weather conditions(wind, visibility, and sky conditions). The PROB30group is used when the occurrence of thunderstormsor precipitation is 30−39% and the PROB40 group isused when the occurrence of thunderstorms orprecipitation is 40−49%. This is followed by twofour−digit groups separated by a “/”, giving thebeginning date and hour, and the ending date and hourof the time period during which the thunderstorms orprecipitation are expected.

NOTE−NWS does not use PROB 40 in the TAF. However U.S.Military generated TAFS may include PROB40. PROB30will not be shown during the first nine hours of a NWSforecast.

EXAMPLE−PROB40 2221/2302 1/2SM +TSRA “chance between

2100Z and 0200Z ofvisibility one−half statute mile in thunderstorms andheavy rain.”

PROB30 3010/3014 1SM RASN “chance between.1000Z and 1400Z ofvisibility one statutemile in mixed rainand snow.”

e. Forecast Change Indicators. The followingchange indicators are used when either a rapid,gradual, or temporary change is expected in some orall of the forecast meteorological conditions. Eachchange indicator marks a time group within the TAFreport.

1. From (FM) group. The FM group is usedwhen a rapid change, usually occurring in less thanone hour, in prevailing conditions is expected.Typically, a rapid change of prevailing conditions tomore or less a completely new set of prevailingconditions is associated with a synoptic featurepassing through the terminal area (cold or warmfrontal passage). Appended to the “FM” indicator isthe six−digit date, hour, and minute the change isexpected to begin and continues until the next changegroup or until the end of the current forecast. A “FM”group will mark the beginning of a new line in a TAFreport (indented 5 spaces). Each “FM” groupcontains all the required elements−wind, visibility,weather, and sky condition. Weather will be omittedin “FM” groups when it is not significant to aviation.FM groups will not include the contraction NSW.

AIM8/15/19


EXAMPLE−FM210100 14010KT P6SM SKC − “after 0100Z on the21st, wind one four zero at one zero, visibility more than six,sky clear.”

2. Becoming (BECMG) group. The BECMGgroup is used when a gradual change in conditions isexpected over a longer time period, usually twohours. The time period when the change is expectedis two four−digit groups separated by a “/”, with thebeginning date and hour, and ending date and hour ofthe change period which follows the BECMGindicator. The gradual change will occur at anunspecified time within this time period. Only thechanging forecast meteorological conditions areincluded in BECMG groups. The omitted conditionsare carried over from the previous time group.NOTE−The NWS does not use BECMG in the TAF.EXAMPLE−OVC012 BECMG 0114/0116 BKN020 − “ceiling onethousand two hundred overcast. Then a gradual changeto ceiling two thousand broken between 1400Z on the 1stand 1600Z on the 1st.”

3. Temporary (TEMPO) group. The TEMPOgroup is used for any conditions in wind, visibility,weather, or sky condition which are expected to lastfor generally less than an hour at a time (occasional),and are expected to occur during less than half thetime period. The TEMPO indicator is followed bytwo four−digit groups separated by a “/”. The firstfour digit group gives the beginning date and hour,and the second four digit group gives the ending dateand hour of the time period during which thetemporary conditions are expected. Only thechanging forecast meteorological conditions areincluded in TEMPO groups. The omitted conditionsare carried over from the previous time group.

EXAMPLE−1. SCT030 TEMPO 0519/0523 BKN030 − “threethousand scattered with occasional ceilings three thousandbroken between 1900Z on the 5th and 2300Z on the 5th.”

2. 4SM HZ TEMPO 1900/1906 2SM BR HZ − “visibilityfour in haze with occasional visibility two in mist and hazebetween 0000Z on the 19th and 0600Z on the 19th.”

AIM10/12/17

7−2−1Altimeter Setting Procedures

Section 2. Altimeter Setting Procedures

7−2−1. General

a. The accuracy of aircraft altimeters is subject tothe following factors:

1. Nonstandard temperatures of the atmosphere.

2. Nonstandard atmospheric pressure.

3. Aircraft static pressure systems (positionerror); and

4. Instrument error.

b. EXTREME CAUTION SHOULD BE EXER-CISED WHEN FLYING IN PROXIMITY TOOBSTRUCTIONS OR TERRAIN IN LOW TEM-PERATURES AND PRESSURES. This is especiallytrue in extremely cold temperatures that cause a largedifferential between the Standard Day temperatureand actual temperature. This circumstance can causeserious errors that result in the aircraft beingsignificantly lower than the indicated altitude.

NOTE−Standard temperature at sea level is 15 degrees Celsius(59 degrees Fahrenheit). The temperature gradient fromsea level is minus 2 degrees Celsius (3.6 degreesFahrenheit) per 1,000 feet. Pilots should apply correctionsfor static pressure systems and/or instruments, ifappreciable errors exist.

c. The adoption of a standard altimeter setting atthe higher altitudes eliminates station barometererrors, some altimeter instrument errors, and errorscaused by altimeter settings derived from differentgeographical sources.

7−2−2. Procedures

The cruising altitude or flight level of aircraft must bemaintained by reference to an altimeter which mustbe set, when operating:

a. Below 18,000 feet MSL.

1. When the barometric pressure is31.00 inches Hg. or less. To the current reportedaltimeter setting of a station along the route andwithin 100 NM of the aircraft, or if there is no stationwithin this area, the current reported altimeter settingof an appropriate available station. When an aircraft

is en route on an instrument flight plan, air trafficcontrollers will furnish this information to the pilot atleast once while the aircraft is in the controllers areaof jurisdiction. In the case of an aircraft not equippedwith a radio, set to the elevation of the departureairport or use an appropriate altimeter settingavailable prior to departure.

2. When the barometric pressure exceeds31.00 inches Hg. The following procedures will beplaced in effect by NOTAM defining the geographicarea affected:

(a) For all aircraft. Set 31.00 inches for enroute operations below 18,000 feet MSL. Maintainthis setting until beyond the affected area or untilreaching final approach segment. At the beginning ofthe final approach segment, the current altimetersetting will be set, if possible. If not possible,31.00 inches will remain set throughout the ap-proach. Aircraft on departure or missed approach willset 31.00 inches prior to reaching any mandatory/crossing altitude or 1,500 feet AGL, whichever islower. (Air traffic control will issue actual altimetersettings and advise pilots to set 31.00 inches in theiraltimeters for en route operations below 18,000 feetMSL in affected areas.)

(b) During preflight, barometric altimetersmust be checked for normal operation to the extentpossible.

(c) For aircraft with the capability of settingthe current altimeter setting and operating intoairports with the capability of measuring the currentaltimeter setting, no additional restrictions apply.

(d) For aircraft operating VFR, there are noadditional restrictions, however, extra diligence inflight planning and in operating in these conditions isessential.

(e) Airports unable to accurately measurebarometric pressures above 31.00 inches of Hg. willreport the barometric pressure as “missing” or “inexcess of 31.00 inches of Hg.” Flight operations toand from those airports are restricted to VFR weatherconditions.

AIM 10/12/17

7−2−2 Altimeter Setting Procedures

(f) For aircraft operating IFR and unable to setthe current altimeter setting, the following restric-tions apply:

(1) To determine the suitability of depar-ture alternate airports, destination airports, anddestination alternate airports, increase ceilingrequirements by 100 feet and visibility requirementsby 1/4 statute mile for each 1/10 of an inch of Hg., orany portion thereof, over 31.00 inches. Theseadjusted values are then applied in accordance withthe requirements of the applicable operatingregulations and operations specifications.

EXAMPLE−Destination altimeter is 31.28 inches, ILS DH 250 feet(200−1/2 ). When flight planning, add 300−3/4 to theweather requirements which would become 500−11/4.

(2) On approach, 31.00 inches will remainset. Decision height (DH) or minimum descentaltitude must be deemed to have been reached whenthe published altitude is displayed on the altimeter.

NOTE−Although visibility is normally the limiting factor on anapproach, pilots should be aware that when reaching DHthe aircraft will be higher than indicated. Using theexample above the aircraft would be approximately300 feet higher.

(3) These restrictions do not apply toauthorized Category II and III ILS operations nor dothey apply to certificate holders using approved QFEaltimetry systems.

(g) The FAA Regional Flight StandardsDivision Manager of the affected area is authorized toapprove temporary waivers to permit emergencyresupply or emergency medical service operation.

b. At or above 18,000 feet MSL. To 29.92 inch-es of mercury (standard setting). The lowest usableflight level is determined by the atmospheric pressurein the area of operation as shown in TBL 7−2−1.

TBL 7−2−1

Lowest Usable Flight Level

Altimeter Setting(Current Reported)

Lowest UsableFlight Level

29.92 or higher 180

29.91 to 29.42 185

29.41 to 28.92 190

28.91 to 28.42 195

28.41 to 27.92 200

c. Where the minimum altitude, as prescribed in14 CFR Section 91.159 and 14 CFR Section 91.177,is above 18,000 feet MSL, the lowest usable flightlevel must be the flight level equivalent of theminimum altitude plus the number of feet specified inTBL 7−2−2.

TBL 7−2−2

Lowest Flight Level Correction Factor

Altimeter Setting Correction Factor

29.92 or higher none

29.91 to 29.42 500 feet

29.41 to 28.92 1000 feet

28.91 to 28.42 1500 feet

28.41 to 27.92 2000 feet

27.91 to 27.42 2500 feet

EXAMPLE−The minimum safe altitude of a route is 19,000 feet MSLand the altimeter setting is reported between 29.92 and29.42 inches of mercury, the lowest usable flight level willbe 195, which is the flight level equivalent of 19,500 feetMSL (minimum altitude plus 500 feet).

AIM10/12/17

7−2−3Altimeter Setting Procedures

7−2−3. Altimeter Errors

a. Most pressure altimeters are subject tomechanical, elastic, temperature, and installationerrors. (Detailed information regarding the use ofpressure altimeters is found in the Instrument FlyingHandbook, Chapter IV.) Although manufacturingand installation specifications, as well as the periodictest and inspections required by regulations (14 CFRPart 43, Appendix E), act to reduce these errors, anyscale error may be observed in the following manner:

1. Set the current reported altimeter setting onthe altimeter setting scale.

2. Altimeter should now read field elevation ifyou are located on the same reference level used toestablish the altimeter setting.

3. Note the variation between the known fieldelevation and the altimeter indication. If this variationis in the order of plus or minus 75 feet, the accuracyof the altimeter is questionable and the problemshould be referred to an appropriately rated repairstation for evaluation and possible correction.

b. Once in flight, it is very important to obtainfrequently current altimeter settings en route. If youdo not reset your altimeter when flying from an areaof high pressure into an area of low pressure, youraircraft will be closer to the surface than youraltimeter indicates. An inch error in the altimetersetting equals 1,000 feet of altitude. To quote an oldsaying: “GOING FROM A HIGH TO A LOW,LOOK OUT BELOW.”

c. Temperature also has an effect on the accuracyof altimeters and your altitude. The crucial values toconsider are standard temperature versus the ambient(at altitude) temperature and the elevation above thealtitude setting reporting source. It is these“differences” that cause the error in indicatedaltitude. When the column of air is warmer thanstandard, you are higher than your altimeter indicates.Subsequently, when the column of air is colder thanstandard, you are lower than indicated. It is themagnitude of these “differences” that determine themagnitude of the error. When flying into a cooler airmass while maintaining a constant indicated altitude,you are losing true altitude. However, flying into acooler air mass does not necessarily mean you will belower than indicated if the difference is still on theplus side. For example, while flying at 10,000 feet(where STANDARD temperature is −5 degrees

Celsius (C)), the outside air temperature cools from+5 degrees C to 0 degrees C, the temperature errorwill nevertheless cause the aircraft to be HIGHERthan indicated. It is the extreme “cold” difference thatnormally would be of concern to the pilot. Also, whenflying in cold conditions over mountainous terrain,the pilot should exercise caution in flight planningboth in regard to route and altitude to ensure adequateen route and terminal area terrain clearance.

NOTE−Non-standard temperatures can result in a change toeffective vertical paths and actual descent rates whileusing aircraft Baro-VNAV equipment for vertical guidanceon final approach segments. A higher than standardtemperature will result in a steeper gradient and increasedactual descent rate. Indications of these differences areoften not directly related to vertical speed indications.Conversely, a lower than standard temperature will resultin a shallower descent gradient and reduced actual descentrate. Pilots should consider potential consequences ofthese effects on approach minimums, power settings, sightpicture, visual cues, etc., especially for high-altitude orterrain-challenged locations and during low-visibilityconditions.

d. TBL 7−2−3, derived from ICAO formulas,indicates how much error can exist when operating incold temperatures. To use the table, find the reportedtemperature in the left column, read across the toprow to locate the height above the airport/reportingstation (i.e., subtract the airport/ reporting elevationfrom the intended flight altitude). The intersection ofthe column and row is how much lower the aircraftmay actually be as a result of the possible coldtemperature induced error.

e. Pilots are responsible to compensate for coldtemperature altimetry errors when operating into anairport with any published cold temperaturerestriction and a reported airport temperature at orbelow the published temperature restriction. Pilotsmust ensure compensating aircraft are correcting onthe proper segment or segments of the approach.Manually correct if compensating aircraft system isinoperable. Pilots manually correcting, are respons-ible to calculate and apply a cold temperature altitudecorrection derived from TBL 7−2−3 to the affectedapproach segment or segments. Pilots must advise thecold temperature altitude correction to Air TrafficControl (ATC). Pilots are not required to advise ATCof a cold temperature altitude correction inside of thefinal approach fix.

AIM 10/12/17

7−2−4 Altimeter Setting Procedures

TBL 7−2−3

ICAO Cold Temperature Error TableR

epor

ted

Tem

p �

C

Height Above Airport in Feet

200 300 400 500 600 700 800 900 1000 1500 2000 3000 4000 5000

+10 10 10 10 10 20 20 20 20 20 30 40 60 80 90

0 20 20 30 30 40 40 50 50 60 90 120 170 230 280

−10 20 30 40 50 60 70 80 90 100 150 200 290 390 490

−20 30 50 60 70 90 100 120 130 140 210 280 420 570 710

−30 40 60 80 100 120 140 150 170 190 280 380 570 760 950

−40 50 80 100 120 150 170 190 220 240 360 480 720 970 1210

−50 60 90 120 150 180 210 240 270 300 450 590 890 1190 1500

EXAMPLE−Temperature−10 degrees Celsius, and the aircraft altitude is 1,000 feet above the airport elevation. The chart shows thatthe reported current altimeter setting may place the aircraft as much as 100 feet below the altitude indicated by the altimeter.

7−2−4. High Barometric Pressure

a. Cold, dry air masses may produce barometricpressures in excess of 31.00 inches of Mercury, andmany altimeters do not have an accurate means ofbeing adjusted for settings of these levels. When thealtimeter cannot be set to the higher pressure setting,the aircraft actual altitude will be higher than thealtimeter indicates.

REFERENCE−AIM, Paragraph 7−2−3 , Altimeter Errors.

b. When the barometric pressure exceeds31.00 inches, air traffic controllers will issue theactual altimeter setting, and:

1. En Route/Arrivals. Advise pilots to remainset on 31.00 inches until reaching the final approachsegment.

2. Departures. Advise pilots to set 31.00 inch-es prior to reaching any mandatory/crossing altitudeor 1,500 feet, whichever is lower.

c. The altimeter error caused by the high pressurewill be in the opposite direction to the error caused bythe cold temperature.

7−2−5. Low Barometric Pressure

When abnormally low barometric pressure condi-tions occur (below 28.00), flight operations byaircraft unable to set the actual altimeter setting arenot recommended.

NOTE−The true altitude of the aircraft is lower than the indicatedaltitude if the pilot is unable to set the actual altimetersetting.

AIM8/15/19

7−3−1Wake Turbulence

Section 3. Wake Turbulence

7−3−1. General

a. Every aircraft generates wake turbulence whilein flight. Wake turbulence is a function of an aircraftproducing lift, resulting in the formation of twocounter−rotating vortices trailing behind the aircraft.

b. Wake turbulence from the generating aircraftcan affect encountering aircraft due to the strength,duration, and direction of the vortices. Waketurbulence can impose rolling moments exceedingthe roll−control authority of encountering aircraft,causing possible injury to occupants and damage toaircraft. Pilots should always be aware of thepossibility of a wake turbulence encounter whenflying through the wake of another aircraft, and adjustthe flight path accordingly.

7−3−2. Vortex Generation

a. The creation of a pressure differential over thewing surface generates lift. The lowest pressureoccurs over the upper wing surface and the highestpressure under the wing. This pressure differentialtriggers the roll up of the airflow at the rear of thewing resulting in swirling air masses trailingdownstream of the wing tips. After the roll up iscompleted, the wake consists of two counter−rotatingcylindrical vortices. (See FIG 7−3−1.) The wakevortex is formed with most of the energy concentratedwithin a few feet of the vortex core.

FIG 7−3−1

Wake Vortex Generation

b. More aircraft are being manufactured orretrofitted with winglets. There are several types ofwinglets, but their primary function is to increase fuelefficiency by improving the lift−to−drag ratio.Studies have shown that winglets have a negligibleeffect on wake turbulence generation, particularlywith the slower speeds involved during departuresand arrivals.

7−3−3. Vortex Strength

a. Weight, speed, wingspan, and shape of thegenerating aircraft’s wing all govern the strength ofthe vortex. The vortex characteristics of any givenaircraft can also be changed by extension of flaps orother wing configuring devices. However, the vortexstrength from an aircraft increases proportionately toan increase in operating weight or a decrease inaircraft speed. Since the turbulence from a “dirty”aircraft configuration hastens wake decay, thegreatest vortex strength occurs when the generatingaircraft is HEAVY, CLEAN, and SLOW.

b. Induced Roll

1. In rare instances, a wake encounter couldcause catastrophic inflight structural damage to anaircraft. However, the usual hazard is associated withinduced rolling moments that can exceed theroll−control authority of the encountering aircraft.During inflight testing, aircraft intentionally flewdirectly up trailing vortex cores of larger aircraft.These tests demonstrated that the ability of aircraft tocounteract the roll imposed by wake vortex dependsprimarily on the wingspan and counter−controlresponsiveness of the encountering aircraft. Thesetests also demonstrated the difficulty of an aircraft toremain within a wake vortex. The natural tendency isfor the circulation to eject aircraft from the vortex.

2. Counter control is usually effective andinduced roll minimal in cases where the wingspanand ailerons of the encountering aircraft extendbeyond the rotational flow field of the vortex. It ismore difficult for aircraft with short wingspan(relative to the generating aircraft) to counter theimposed roll induced by vortex flow. Pilots of shortspan aircraft, even of the high performance type, mustbe especially alert to vortex encounters. (See FIG 7−3−2.)

AIM 8/15/19

7−3−2 Wake Turbulence

FIG 7−3−2

Wake Encounter Counter Control

COUNTERCONTROL

7−3−4. Vortex Behavior

a. Trailing vortices have certain behavioralcharacteristics which can help a pilot visualize thewake location and thereby take avoidance precau-tions.

1. An aircraft generates vortices from themoment it rotates on takeoff to touchdown, sincetrailing vortices are a by−product of wing lift. Prior totakeoff or touchdown pilots should note the rotationor touchdown point of the preceding aircraft. (SeeFIG 7−3−3.)

2. The vortex circulation is outward, upwardand around the wing tips when viewed from eitherahead or behind the aircraft. Tests with larger aircrafthave shown that the vortices remain spaced a bit lessthan a wingspan apart, drifting with the wind, ataltitudes greater than a wingspan from the ground. Inview of this, if persistent vortex turbulence isencountered, a slight change of altitude (upward) andlateral position (upwind) should provide a flight pathclear of the turbulence.

3. Flight tests have shown that the vortices fromlarger aircraft sink at a rate of several hundred feet perminute, slowing their descent and diminishing instrength with time and distance behind the generatingaircraft. Pilots should fly at or above the precedingaircraft’s flight path, altering course as necessary toavoid the area directly behind and below thegenerating aircraft. (See FIG 7−3−4.) Pilots, in allphases of flight, must remain vigilant of possiblewake effects created by other aircraft. Studies haveshown that atmospheric turbulence hastens wakebreakup, while other atmospheric conditions cantransport wake horizontally and vertically.

4. When the vortices of larger aircraft sink closeto the ground (within 100 to 200 feet), they tend tomove laterally over the ground at a speed of 2 or3 knots. (See FIG 7−3−5.)

FIG 7−3−3

Wake Ends/Wake Begins

Touchdown Rotation

Wake Ends Wake Begins

AIM8/15/19


FIG 7−3−4

Vortex Flow Field

AVOIDAVOIDNominally 500-1000 Ft.Nominally 500-1000 Ft.

Sink RateSeveral Hundred Ft.,/Min.

Sink RateSeveral Hundred Ft.,/Min.

FIG 7−3−5

Vortex Movement Near Ground − No Wind

No WindNo Wind3K3K 3K3K

FIG 7−3−6

Vortex Movement Near Ground − with Cross Winds

6K6K

(3K + 3K)(3K + 3K)

3K Wind3K Wind

0 (3K - 3K)0 (3K - 3K)

AIM 8/15/19


5. Pilots should be alert at all times for possiblewake vortex encounters when conducting approachand landing operations. The pilot is ultimatelyresponsible for maintaining an appropriate interval,and should consider all available information inpositioning the aircraft in the terminal area, to avoidthe wake turbulence created by a preceding aircraft.Test data shows that vortices can rise with the air massin which they are embedded. The effects of windshear can cause vortex flow field “tilting.” Inaddition, ambient thermal lifting and orographiceffects (rising terrain or tree lines) can cause a vortexflow field to rise and possibly bounce.

b. A crosswind will decrease the lateral movement

of the upwind vortex and increase the movement ofthe downwind vortex. Thus, a light wind with across−runway component of 1 to 5 knots could resultin the upwind vortex remaining in the touchdownzone for a period of time and hasten the drift of thedownwind vortex toward another runway. (SeeFIG 7−3−6.) Similarly, a tailwind condition can movethe vortices of the preceding aircraft forward into thetouchdown zone. THE LIGHT QUARTERINGTAILWIND REQUIRES MAXIMUM CAUTION.Pilots should be alert to large aircraft upwind fromtheir approach and takeoff flight paths. (SeeFIG 7−3−7.)

FIG 7−3−7

Vortex Movement in Ground Effect − Tailwind

Light QuarteringTailwind

Light QuarteringTailwind

x

Tail WindTail Wind

Touchdown PointTouchdown Point

7−3−5. Operations Problem Areas

a. A wake turbulence encounter can range fromnegligible to catastrophic. The impact of theencounter depends on the weight, wingspan, size ofthe generating aircraft, distance from the generatingaircraft, and point of vortex encounter. Theprobability of induced roll increases when theencountering aircraft’s heading is generally alignedwith the flight path of the generating aircraft.

b. AVOID THE AREA BELOW AND BEHINDTHE WAKE GENERATING AIRCRAFT, ESPE-CIALLY AT LOW ALTITUDE WHERE EVEN AMOMENTARY WAKE ENCOUNTER COULD BECATASTROPHIC.

NOTE−A common scenario for a wake encounter is in terminalairspace after accepting clearance for a visual approachbehind landing traffic. Pilots must be cognizant of theirposition relative to the traffic and use all means of vertical

AIM8/15/19


guidance to ensure they do not fly below the flight path ofthe wake generating aircraft.

c. Pilots should be particularly alert in calm windconditions and situations where the vortices could:

1. Remain in the touchdown area.

2. Drift from aircraft operating on a nearbyrunway.

3. Sink into the takeoff or landing path from acrossing runway.

4. Sink into the traffic pattern from other airportoperations.

5. Sink into the flight path of VFR aircraftoperating on the hemispheric altitude 500 feet below.

d. Pilots should attempt to visualize the vortextrail of aircraft whose projected flight path they mayencounter. When possible, pilots of larger aircraftshould adjust their flight paths to minimize vortexexposure to other aircraft.

7−3−6. Vortex Avoidance Procedures

a. Under certain conditions, airport traffic control-lers apply procedures for separating IFR aircraft. If apilot accepts a clearance to visually follow apreceding aircraft, the pilot accepts responsibility forseparation and wake turbulence avoidance. Thecontrollers will also provide to VFR aircraft, withwhom they are in communication and which in thetower’s opinion may be adversely affected by waketurbulence from a larger aircraft, the position, altitudeand direction of flight of larger aircraft followed bythe phrase “CAUTION − WAKE TURBULENCE.”After issuing the caution for wake turbulence, theairport traffic controllers generally do not provideadditional information to the following aircraftunless the airport traffic controllers know thefollowing aircraft is overtaking the precedingaircraft. WHETHER OR NOT A WARNING ORINFORMATION HAS BEEN GIVEN, HOWEVER,THE PILOT IS EXPECTED TO ADJUST AIR-CRAFT OPERATIONS AND FLIGHT PATH ASNECESSARY TO PRECLUDE SERIOUS WAKEENCOUNTERS. When any doubt exists aboutmaintaining safe separation distances betweenaircraft during approaches, pilots should ask thecontrol tower for updates on separation distance andaircraft groundspeed.

b. The following vortex avoidance procedures arerecommended for the various situations:

1. Landing behind a larger aircraft− samerunway. Stay at or above the larger aircraft’s finalapproach flight path−note its touchdown point−landbeyond it.

2. Landing behind a larger aircraft− whenparallel runway is closer than 2,500 feet. Considerpossible drift to your runway. Stay at or above thelarger aircraft’s final approach flight path− note itstouchdown point.

3. Landing behind a larger aircraft− crossingrunway. Cross above the larger aircraft’s flight path.

4. Landing behind a departing larger air-craft− same runway. Note the larger aircraft’srotation point− land well prior to rotation point.

5. Landing behind a departing larger air-craft− crossing runway. Note the larger aircraft’srotation point− if past the intersection− continue theapproach− land prior to the intersection. If largeraircraft rotates prior to the intersection, avoid flightbelow the larger aircraft’s flight path. Abandon theapproach unless a landing is ensured well beforereaching the intersection.

6. Departing behind a larger aircraft. Notethe larger aircraft’s rotation point and rotate prior tothe larger aircraft’s rotation point. Continue climbingabove the larger aircraft’s climb path until turningclear of the larger aircraft’s wake. Avoid subsequentheadings which will cross below and behind a largeraircraft. Be alert for any critical takeoff situationwhich could lead to a vortex encounter.

7. Intersection takeoffs− same runway. Bealert to adjacent larger aircraft operations, particular-ly upwind of your runway. If intersection takeoffclearance is received, avoid subsequent headingwhich will cross below a larger aircraft’s path.

8. Departing or landing after a largeraircraft executing a low approach, missedapproach, or touch−and−go landing. Becausevortices settle and move laterally near the ground, thevortex hazard may exist along the runway and in yourflight path after a larger aircraft has executed a lowapproach, missed approach, or a touch−and−golanding, particular in light quartering wind condi-tions. You should ensure that an interval of at least2 minutes has elapsed before your takeoff or landing.

AIM 8/15/19


9. En route VFR (thousand−foot altitude plus500 feet). Avoid flight below and behind a largeaircraft’s path. If a larger aircraft is observed above onthe same track (meeting or overtaking) adjust yourposition laterally, preferably upwind.

7−3−7. Helicopters

In a slow hover taxi or stationary hover near thesurface, helicopter main rotor(s) generate downwashproducing high velocity outwash vortices to adistance approximately three times the diameter ofthe rotor. When rotor downwash hits the surface, theresulting outwash vortices have behavioral character-istics similar to wing tip vortices produced by fixedwing aircraft. However, the vortex circulation isoutward, upward, around, and away from the mainrotor(s) in all directions. Pilots of small aircraftshould avoid operating within three rotor diametersof any helicopter in a slow hover taxi or stationaryhover. In forward flight, departing or landinghelicopters produce a pair of strong, high−speedtrailing vortices similar to wing tip vortices of largerfixed wing aircraft. Pilots of small aircraft should usecaution when operating behind or crossing behindlanding and departing helicopters.

7−3−8. Pilot Responsibility

a. Research and testing have been conducted, inaddition to ongoing wake initiatives, in an attempt tomitigate the effects of wake turbulence. Pilots mustexercise vigilance in situations where they areresponsible for avoiding wake turbulence.

b. Pilots are reminded that in operations con-ducted behind all aircraft, acceptance of instructionsfrom ATC in the following situations is anacknowledgment that the pilot will ensure safetakeoff and landing intervals and accepts theresponsibility for providing wake turbulence separa-tion.

1. Traffic information.

2. Instructions to follow an aircraft; and

3. The acceptance of a visual approachclearance.

c. For operations conducted behind super orheavy aircraft, ATC will specify the word “super” or“heavy” as appropriate, when this information is

known. Pilots of super or heavy aircraft shouldalways use the word “super” or “heavy” in radiocommunications.

d. Super, heavy, and large jet aircraft operatorsshould use the following procedures during anapproach to landing. These procedures establish adependable baseline from which pilots of in−trail,lighter aircraft may reasonably expect to makeeffective flight path adjustments to avoid seriouswake vortex turbulence.

1. Pilots of aircraft that produce strong wakevortices should make every attempt to fly on theestablished glidepath, not above it; or, if glidepathguidance is not available, to fly as closely as possibleto a “3−1” glidepath, not above it.

EXAMPLE−Fly 3,000 feet at 10 miles from touchdown, 1,500 feet at 5miles, 1,200 feet at 4 miles, and so on to touchdown.

2. Pilots of aircraft that produce strong wakevortices should fly as closely as possible to theapproach course centerline or to the extendedcenterline of the runway of intended landing asappropriate to conditions.

e. Pilots operating lighter aircraft on visualapproaches in−trail to aircraft producing strong wakevortices should use the following procedures to assistin avoiding wake turbulence. These procedures applyonly to those aircraft that are on visual approaches.

1. Pilots of lighter aircraft should fly on orabove the glidepath. Glidepath reference may befurnished by an ILS, by a visual approach slopesystem, by other ground−based approach slopeguidance systems, or by other means. In the absenceof visible glidepath guidance, pilots may very nearlyduplicate a 3−degree glideslope by adhering to the“3 to 1” glidepath principle.

EXAMPLE−Fly 3,000 feet at 10 miles from touchdown, 1,500 feet at5 miles, 1,200 feet at 4 miles, and so on to touchdown.

2. If the pilot of the lighter following aircraft hasvisual contact with the preceding heavier aircraft andalso with the runway, the pilot may further adjust forpossible wake vortex turbulence by the followingpractices:

(a) Pick a point of landing no less than1,000 feet from the arrival end of the runway.

AIM8/15/19


(b) Establish a line−of−sight to that landingpoint that is above and in front of the heavierpreceding aircraft.

(c) When possible, note the point of landingof the heavier preceding aircraft and adjust point ofintended landing as necessary.

EXAMPLE−A puff of smoke may appear at the 1,000−foot markings ofthe runway, showing that touchdown was that point;therefore, adjust point of intended landing to the1,500−foot markings.

(d) Maintain the line−of−sight to the point ofintended landing above and ahead of the heavierpreceding aircraft; maintain it to touchdown.

(e) Land beyond the point of landing of thepreceding heavier aircraft. Ensure you have adequaterunway remaining, if conducting a touch−and−golanding, or adequate stopping distance available fora full stop landing.

f. During visual approaches pilots may ask ATCfor updates on separation and groundspeed withrespect to heavier preceding aircraft, especially whenthere is any question of safe separation from waketurbulence.

g. Pilots should notify ATC when a wake event isencountered. Be as descriptive as possible (i.e., bankangle, altitude deviations, intensity and duration ofevent, etc.) when reporting the event. ATC will recordthe event through their reporting system. You are alsoencouraged to use the Aviation Safety ReportingSystem (ASRS) to report wake events.

7−3−9. Air Traffic Wake TurbulenceSeparations

a. Because of the possible effects of waketurbulence, controllers are required to apply no lessthan minimum required separation to all aircraftoperating behind a Super or Heavy, and to Smallaircraft operating behind a B757, when aircraft areIFR; VFR and receiving Class B, Class C, or TRSAairspace services; or VFR and being radar sequenced.

1. Separation is applied to aircraft operatingdirectly behind a super or heavy at the same altitudeor less than 1,000 feet below, and to small aircraftoperating directly behind a B757 at the same altitudeor less than 500 feet below:

(a) Heavy behind super − 6 miles.

(b) Large behind super − 7 miles.

(c) Small behind super − 8 miles.

(d) Heavy behind heavy −4 miles.

(e) Small/large behind heavy − 5 miles.

(f) Small behind B757 − 4 miles.

2. Also, separation, measured at the time thepreceding aircraft is over the landing threshold, isprovided to small aircraft:

(a) Small landing behind heavy − 6 miles.

(b) Small landing behind large, non−B757− 4 miles.REFERENCE−Pilot/Controller Glossary Term− Aircraft Classes.

b. Additionally, appropriate time or distanceintervals are provided to departing aircraft when thedeparture will be from the same threshold, a parallelrunway separated by less than 2,500 feet with lessthan 500 feet threshold stagger, or on a crossingrunway and projected flight paths will cross:

1. Three minutes or the appropriate radarseparation when takeoff will be behind a superaircraft;

2. Two minutes or the appropriate radarseparation when takeoff will be behind a heavyaircraft.

3. Two minutes or the appropriate radarseparation when a small aircraft will takeoff behinda B757.

NOTE−Controllers may not reduce or waive these intervals.

c. A 3−minute interval will be provided when asmall aircraft will takeoff:

1. From an intersection on the same runway(same or opposite direction) behind a departing largeaircraft (except B757), or

2. In the opposite direction on the same runwaybehind a large aircraft (except B757) takeoff orlow/missed approach.

NOTE−This 3−minute interval may be waived upon specific pilotrequest.

d. A 3−minute interval will be provided when asmall aircraft will takeoff:

1. From an intersection on the same runway(same or opposite direction) behind a departingB757, or

AIM 8/15/19


2. In the opposite direction on the same runwaybehind a B757 takeoff or low/missed approach.

NOTE−This 3−minute interval may not be waived.

e. A 4−minute interval will be provided for allaircraft taking off behind a super aircraft, and a3−minute interval will be provided for all aircrafttaking off behind a heavy aircraft when the operationsare as described in subparagraphs c1 and c2 above,and are conducted on either the same runway orparallel runways separated by less than 2,500 feet.Controllers may not reduce or waive this interval.

f. Pilots may request additional separation (i.e.,2 minutes instead of 4 or 5 miles) for wake turbulenceavoidance. This request should be made as soon aspractical on ground control and at least before taxiingonto the runway.

NOTE−14 CFR Section 91.3(a) states: “The pilot−in−command ofan aircraft is directly responsible for and is the finalauthority as to the operation of that aircraft.”

g. Controllers may anticipate separation and neednot withhold a takeoff clearance for an aircraftdeparting behind a large, heavy, or super aircraft ifthere is reasonable assurance the required separationwill exist when the departing aircraft starts takeoffroll.

NOTE−With the advent of new wake turbulence separationmethodologies known as Wake Turbulence Recategoriza-

tion, some of the requirements listed above may vary atfacilities authorized to operate in accordance with WakeTurbulence Recategorization directives.

REFERENCE−FAA Order JO 7110.659 Wake Turbulence RecategorizationFAA Order JO 7110.123 Wake Turbulence Recategorization − Phase IIFAA Order JO 7110.126, Consolidated Wake Turbulence

7−3−10. Development and New Capabilities

a. The suite of available wake turbulence tools,rules, and procedures is expanding, with thedevelopment of new methodologies. Based onextensive analysis of wake vortex behavior, newprocedures and separation standards are beingdeveloped and implemented in the US andthroughout the world. Wake research involves thewake generating aircraft as well as the waketoleration of the trailing aircraft.

b. The FAA and ICAO are leading initiatives, interminal environments, to implement next−genera-tion wake turbulence procedures and separationstandards. The FAA has undertaken an effort torecategorize the existing fleet of aircraft and modifyassociated wake turbulence separation minima. Thisinitiative is termed Wake Turbulence Recategoriza-tion (RECAT), and changes the current weight−basedclasses (Super, Heavy, B757, Large, Small+, andSmall) to a wake−based categorical system thatutilizes the aircraft matrices of weight, wingspan, andapproach speed. RECAT is currently in use at alimited number of airports in the National AirspaceSystem.

AIM8/15/19

7−4−1Bird Hazards and Flight Over National Refuges, Parks, and Forests

Section 4. Bird Hazards and Flight Over NationalRefuges, Parks, and Forests

7−4−1. Migratory Bird Activity

a. Bird strike risk increases because of birdmigration during the months of March through April,and August through November.

b. The altitudes of migrating birds vary with windsaloft, weather fronts, terrain elevations, cloudconditions, and other environmental variables. Whileover 90 percent of the reported bird strikes occur at orbelow 3,000 feet AGL, strikes at higher altitudes arecommon during migration. Ducks and geese arefrequently observed up to 7,000 feet AGL and pilotsare cautioned to minimize en route flying at loweraltitudes during migration.

c. Considered the greatest potential hazard toaircraft because of their size, abundance, or habit offlying in dense flocks are gulls, waterfowl, vultures,hawks, owls, egrets, blackbirds, and starlings.Four major migratory flyways exist in the U.S. TheAtlantic flyway parallels the Atlantic Coast. TheMississippi Flyway stretches from Canada throughthe Great Lakes and follows the Mississippi River.The Central Flyway represents a broad area east of theRockies, stretching from Canada through CentralAmerica. The Pacific Flyway follows the west coastand overflies major parts of Washington, Oregon, andCalifornia. There are also numerous smaller flywayswhich cross these major north-south migratoryroutes.

7−4−2. Reducing Bird Strike Risks

a. The most serious strikes are those involvingingestion into an engine (turboprops and turbine jetengines) or windshield strikes. These strikes canresult in emergency situations requiring promptaction by the pilot.

b. Engine ingestions may result in sudden loss ofpower or engine failure. Review engine outprocedures, especially when operating from airportswith known bird hazards or when operating near highbird concentrations.

c. Windshield strikes have resulted in pilotsexperiencing confusion, disorientation, loss ofcommunications, and aircraft control problems.Pilots are encouraged to review their emergencyprocedures before flying in these areas.

d. When encountering birds en route, climb toavoid collision, because birds in flocks generallydistribute themselves downward, with lead birdsbeing at the highest altitude.

e. Avoid overflight of known areas of birdconcentration and flying at low altitudes during birdmigration. Charted wildlife refuges and other naturalareas contain unusually high local concentration ofbirds which may create a hazard to aircraft.

7−4−3. Reporting Bird Strikes

Pilots are urged to report any bird or other wildlifestrike using FAA Form 5200−7, Bird/Other WildlifeStrike Report (Appendix 1). Additional forms areavailable at any FSS; at any FAA Regional Office orat https://www.faa.gov/airports/airport_safety/wildlife/. The data derived from these reports are usedto develop standards to cope with this potentialhazard to aircraft and for documentation of necessaryhabitat control on airports.

7−4−4. Reporting Bird and Other WildlifeActivities

If you observe birds or other animals on or near therunway, request airport management to disperse thewildlife before taking off. Also contact the nearestFAA ARTCC, FSS, or tower (including non−Federaltowers) regarding large flocks of birds and report the:

a. Geographic location.

b. Bird type (geese, ducks, gulls, etc.).

c. Approximate numbers.

d. Altitude.

e. Direction of bird flight path.

AIM 8/15/19

7−4−2 Bird Hazards and Flight Over National Refuges, Parks, and Forests

7−4−5. Pilot Advisories on Bird and OtherWildlife Hazards

Many airports advise pilots of other wildlife hazardscaused by large animals on the runway through theChart Supplement U.S. and the NOTAM system.Collisions of landing and departing aircraft andanimals on the runway are increasing and are notlimited to rural airports. These accidents have alsooccurred at several major airports. Pilots shouldexercise extreme caution when warned of thepresence of wildlife on and in the vicinity of airports.If you observe deer or other large animals in closeproximity to movement areas, advise the FSS, tower,or airport management.

7−4−6. Flights Over Charted U.S. WildlifeRefuges, Parks, and Forest Service Areas

a. The landing of aircraft is prohibited on lands orwaters administered by the National Park Service,U.S. Fish and Wildlife Service, or U.S. Forest Servicewithout authorization from the respective agency.Exceptions include:

1. When forced to land due to an emergencybeyond the control of the operator;

2. At officially designated landing sites; or

3. An approved official business of the FederalGovernment.

b. Pilots are requested to maintain a minimumaltitude of 2,000 feet above the surface of thefollowing: National Parks, Monuments, Seashores,

Lakeshores, Recreation Areas and Scenic Riverwaysadministered by the National Park Service, NationalWildlife Refuges, Big Game Refuges, Game Rangesand Wildlife Ranges administered by the U.S. Fishand Wildlife Service, and Wilderness and Primitiveareas administered by the U.S. Forest Service.

NOTE−FAA Advisory Circular AC 91−36, Visual FlightRules (VFR) Flight Near Noise-Sensitive Areas, definesthe surface of a national park area (including parks,forests, primitive areas, wilderness areas, recreationalareas, national seashores, national monuments, nationallakeshores, and national wildlife refuge and range areas)as: the highest terrain within 2,000 feet laterally of theroute of flight, or the upper-most rim of a canyon or valley.

c. Federal statutes prohibit certain types of flightactivity and/or provide altitude restrictions overdesignated U.S. Wildlife Refuges, Parks, and ForestService Areas. These designated areas, for example:Boundary Waters Canoe Wilderness Areas,Minnesota; Haleakala National Park, Hawaii;Yosemite National Park, California; and GrandCanyon National Park, Arizona, are charted onSectional Charts.

d. Federal regulations also prohibit airdrops byparachute or other means of persons, cargo, or objectsfrom aircraft on lands administered by the threeagencies without authorization from the respectiveagency. Exceptions include:

1. Emergencies involving the safety of humanlife; or

2. Threat of serious property loss.

AIM8/15/19

7−5−1Potential Flight Hazards

Section 5. Potential Flight Hazards

7−5−1. Accident Cause Factors

a. The 10 most frequent cause factors for generalaviation accidents that involve the pilot-in-commandare:

1. Inadequate preflight preparation and/orplanning.

2. Failure to obtain and/or maintain flyingspeed.

3. Failure to maintain direction control.

4. Improper level off.

5. Failure to see and avoid objects orobstructions.

6. Mismanagement of fuel.

7. Improper inflight decisions or planning.

8. Misjudgment of distance and speed.

9. Selection of unsuitable terrain.

10. Improper operation of flight controls.

b. This list remains relatively stable and points outthe need for continued refresher training to establisha higher level of flight proficiency for all pilots. Apart of the FAA’s continuing effort to promoteincreased aviation safety is the Aviation SafetyProgram. For information on Aviation SafetyProgram activities contact your nearest FlightStandards District Office.

c. Alertness. Be alert at all times, especiallywhen the weather is good. Most pilots pay attentionto business when they are operating in full IFRweather conditions, but strangely, air collisionsalmost invariably have occurred under ideal weatherconditions. Unlimited visibility appears to encouragea sense of security which is not at all justified.Considerable information of value may be obtainedby listening to advisories being issued in the terminalarea, even though controller workload may prevent apilot from obtaining individual service.

d. Giving Way. If you think another aircraft is tooclose to you, give way instead of waiting for the otherpilot to respect the right-of-way to which you may be

entitled. It is a lot safer to pursue the right-of-wayangle after you have completed your flight.

7−5−2. VFR in Congested Areas

A high percentage of near midair collisions occurbelow 8,000 feet AGL and within 30 miles of anairport. When operating VFR in these highlycongested areas, whether you intend to land at anairport within the area or are just flying through, it isrecommended that extra vigilance be maintained andthat you monitor an appropriate control frequency.Normally the appropriate frequency is an approachcontrol frequency. By such monitoring action you can“get the picture” of the traffic in your area. When theapproach controller has radar, radar traffic advisoriesmay be given to VFR pilots upon request.

REFERENCE−AIM, Paragraph 4−1−15 , Radar Traffic Information Service

7−5−3. Obstructions To Flight

a. General. Many structures exist that couldsignificantly affect the safety of your flight whenoperating below 500 feet AGL, and particularlybelow 200 feet AGL. While 14 CFR Part 91.119allows flight below 500 AGL when over sparselypopulated areas or open water, such operations arevery dangerous. At and below 200 feet AGL there arenumerous power lines, antenna towers, etc., that arenot marked and lighted as obstructions and; therefore,may not be seen in time to avoid a collision. Noticesto Airmen (NOTAMs) are issued on those lightedstructures experiencing temporary light outages.However, some time may pass before the FAA isnotified of these outages, and the NOTAM issued,thus pilot vigilance is imperative.

b. Antenna Towers. Extreme caution should beexercised when flying less than 2,000 feet AGLbecause of numerous skeletal structures, such as radioand television antenna towers, that exceed 1,000 feetAGL with some extending higher than 2,000 feetAGL. Most skeletal structures are supported by guywires which are very difficult to see in good weatherand can be invisible at dusk or during periods ofreduced visibility. These wires can extend about1,500 feet horizontally from a structure; therefore, allskeletal structures should be avoided horizontally by

AIM 8/15/19

7−5−2 Potential Flight Hazards

at least 2,000 feet. Additionally, new towers may notbe on your current chart because the information wasnot received prior to the printing of the chart.

c. Overhead Wires. Overhead transmission andutility lines often span approaches to runways,natural flyways such as lakes, rivers, gorges, andcanyons, and cross other landmarks pilots frequentlyfollow such as highways, railroad tracks, etc. As withantenna towers, these high voltage/power lines or thesupporting structures of these lines may not always bereadily visible and the wires may be virtuallyimpossible to see under certain conditions. In somelocations, the supporting structures of overheadtransmission lines are equipped with unique sequenceflashing white strobe light systems to indicate thatthere are wires between the structures. However,many power lines do not require notice to the FAAand, therefore, are not marked and/or lighted. Manyof those that do require notice do not exceed 200 feetAGL or meet the Obstruction Standard of 14 CFRPart 77 and, therefore, are not marked and/or lighted.All pilots are cautioned to remain extremely vigilantfor these power lines or their supporting structureswhen following natural flyways or during theapproach and landing phase. This is particularlyimportant for seaplane and/or float equipped aircraftwhen landing on, or departing from, unfamiliar lakesor rivers.

d. Other Objects/Structures. There are otherobjects or structures that could adversely affect yourflight such as construction cranes near an airport,newly constructed buildings, new towers, etc. Manyof these structures do not meet charting requirementsor may not yet be charted because of the chartingcycle. Some structures do not require obstructionmarking and/or lighting and some may not be markedand lighted even though the FAA recommended it.

7−5−4. Avoid Flight Beneath UnmannedBalloons

a. The majority of unmanned free balloonscurrently being operated have, extending belowthem, either a suspension device to which the payloador instrument package is attached, or a trailing wireantenna, or both. In many instances these balloonsubsystems may be invisible to the pilot until theaircraft is close to the balloon, thereby creating apotentially dangerous situation. Therefore, goodjudgment on the part of the pilot dictates that aircraft

should remain well clear of all unmanned freeballoons and flight below them should be avoided atall times.

b. Pilots are urged to report any unmanned freeballoons sighted to the nearest FAA ground facilitywith which communication is established. Suchinformation will assist FAA ATC facilities to identifyand flight follow unmanned free balloons operatingin the airspace.

7−5−5. Unmanned Aircraft Systems

a. Unmanned Aircraft Systems (UAS), formerlyreferred to as “Unmanned Aerial Vehicles” (UAVs)or “drones,” are having an increasing operationalpresence in the NAS. Once the exclusive domain ofthe military, UAS are now being operated by variousentities. Although these aircraft are “unmanned,”UAS are flown by a remotely located pilot and crew.Physical and performance characteristics of un-manned aircraft (UA) vary greatly and unlike modelaircraft that typically operate lower than 400 feetAGL, UA may be found operating at virtually anyaltitude and any speed. Sizes of UA can be as smallas several pounds to as large as a commercialtransport aircraft. UAS come in various categoriesincluding airplane, rotorcraft, powered−lift (tilt−rotor), and lighter−than−air. Propulsion systems ofUAS include a broad range of alternatives frompiston powered and turbojet engines to battery andsolar−powered electric motors.

b. To ensure segregation of UAS operations fromother aircraft, the military typically conducts UASoperations within restricted or other special useairspace. However, UAS operations are now beingapproved in the NAS outside of special use airspacethrough the use of FAA−issued Certificates of Waiveror Authorization (COA) or through the issuance of aspecial airworthiness certificate. COA and specialairworthiness approvals authorize UAS flightoperations to be contained within specific geographicboundaries and altitudes, usually require coordina-tion with an ATC facility, and typically require theissuance of a NOTAM describing the operation to beconducted. UAS approvals also require observers toprovide “see−and−avoid” capability to the UAS crewand to provide the necessary compliance with 14 CFRSection 91.113. For UAS operations approved at orabove FL180, UAS operate under the samerequirements as that of manned aircraft (i.e., flights

AIM8/15/19


are operated under instrument flight rules, are incommunication with ATC, and are appropriatelyequipped).

c. UAS operations may be approved at eithercontrolled or uncontrolled airports and are typicallydisseminated by NOTAM. In all cases, approvedUAS operations must comply with all applicableregulations and/or special provisions specified in theCOA or in the operating limitations of the specialairworthiness certificate. At uncontrolled airports,UAS operations are advised to operate well clear ofall known manned aircraft operations. Pilots ofmanned aircraft are advised to follow normaloperating procedures and are urged to monitor theCTAF for any potential UAS activity. At controlledairports, local ATC procedures may be in place tohandle UAS operations and should not require anyspecial procedures from manned aircraft entering ordeparting the traffic pattern or operating in thevicinity of the airport.

d. In addition to approved UAS operationsdescribed above, a recently approved agreementbetween the FAA and the Department of Defenseauthorizes small UAS operations wholly containedwithin Class G airspace, and in no instance, greaterthan 1200 feet AGL over military owned or leasedproperty. These operations do not require any specialauthorization as long as the UA remains within thelateral boundaries of the military installation as wellas other provisions including the issuance of aNOTAM. Unlike special use airspace, these areasmay not be depicted on an aeronautical chart.

e. There are several factors a pilot should considerregarding UAS activity in an effort to reducepotential flight hazards. Pilots are urged to exerciseincreased vigilance when operating in the vicinity ofrestricted or other special use airspace, militaryoperations areas, and any military installation. Areaswith a preponderance of UAS activity are typicallynoted on sectional charts advising pilots of thisactivity. Since the size of a UA can be very small, theymay be difficult to see and track. If a UA isencountered during flight, as with manned aircraft,never assume that the pilot or crew of the UAS can seeyou, maintain increased vigilance with the UA andalways be prepared for evasive action if necessary.Always check NOTAMs for potential UAS activityalong the intended route of flight and exerciseincreased vigilance in areas specified in the NOTAM.

7−5−6. Mountain Flying

a. Your first experience of flying over mountain-ous terrain (particularly if most of your flight time hasbeen over the flatlands of the midwest) could be anever-to-be-forgotten nightmare if proper planning isnot done and if you are not aware of the potentialhazards awaiting. Those familiar section lines are notpresent in the mountains; those flat, level fields forforced landings are practically nonexistent; abruptchanges in wind direction and velocity occur; severeupdrafts and downdrafts are common, particularlynear or above abrupt changes of terrain such as cliffsor rugged areas; even the clouds look different andcan build up with startling rapidity. Mountain flyingneed not be hazardous if you follow the recommenda-tions below.

b. File a Flight Plan. Plan your route to avoidtopography which would prevent a safe forcedlanding. The route should be over populated areas andwell known mountain passes. Sufficient altitudeshould be maintained to permit gliding to a safelanding in the event of engine failure.

c. Don’t fly a light aircraft when the winds aloft, atyour proposed altitude, exceed 35 miles per hour.Expect the winds to be of much greater velocity overmountain passes than reported a few miles from them.Approach mountain passes with as much altitude aspossible. Downdrafts of from 1,500 to 2,000 feet perminute are not uncommon on the leeward side.

d. Don’t fly near or above abrupt changes interrain. Severe turbulence can be expected, especiallyin high wind conditions.

e. Understand Mountain Obscuration. Theterm Mountain Obscuration (MTOS) is used todescribe a visibility condition that is distinguishedfrom IFR because ceilings, by definition, aredescribed as “above ground level” (AGL). Inmountainous terrain clouds can form at altitudessignificantly higher than the weather reportingstation and at the same time nearby mountaintopsmay be obscured by low visibility. In these areas theground level can also vary greatly over a small area.Beware if operating VFR−on−top. You could beoperating closer to the terrain than you think becausethe tops of mountains are hidden in a cloud deckbelow. MTOS areas are identified daily on TheAviation Weather Center located at:http://www.aviationweather.gov.

AIM 8/15/19


f. Some canyons run into a dead end. Don’t fly sofar up a canyon that you get trapped. ALWAYS BEABLE TO MAKE A 180 DEGREE TURN!

g. VFR flight operations may be conducted atnight in mountainous terrain with the application ofsound judgment and common sense. Proper pre-flightplanning, giving ample consideration to winds andweather, knowledge of the terrain and pilotexperience in mountain flying are prerequisites forsafety of flight. Continuous visual contact with thesurface and obstructions is a major concern and flightoperations under an overcast or in the vicinity ofclouds should be approached with extreme caution.

h. When landing at a high altitude field, the sameindicated airspeed should be used as at low elevationfields. Remember: that due to the less dense air ataltitude, this same indicated airspeed actually resultsin higher true airspeed, a faster landing speed, andmore important, a longer landing distance. Duringgusty wind conditions which often prevail at highaltitude fields, a power approach and power landingis recommended. Additionally, due to the fastergroundspeed, your takeoff distance will increaseconsiderably over that required at low altitudes.

i. Effects of Density Altitude. Performancefigures in the aircraft owner’s handbook for length oftakeoff run, horsepower, rate of climb, etc., aregenerally based on standard atmosphere conditions(59 degrees Fahrenheit (15 degrees Celsius), pressure29.92 inches of mercury) at sea level. However,inexperienced pilots, as well as experienced pilots,may run into trouble when they encounter analtogether different set of conditions. This isparticularly true in hot weather and at higherelevations. Aircraft operations at altitudes above sealevel and at higher than standard temperatures arecommonplace in mountainous areas. Such operationsquite often result in a drastic reduction of aircraftperformance capabilities because of the changing airdensity. Density altitude is a measure of air density.It is not to be confused with pressure altitude, truealtitude or absolute altitude. It is not to be used as aheight reference, but as a determining criteria in theperformance capability of an aircraft. Air density

decreases with altitude. As air density decreases,density altitude increases. The further effects of hightemperature and high humidity are cumulative,resulting in an increasing high density altitudecondition. High density altitude reduces all aircraftperformance parameters. To the pilot, this means thatthe normal horsepower output is reduced, propellerefficiency is reduced and a higher true airspeed isrequired to sustain the aircraft throughout itsoperating parameters. It means an increase in runwaylength requirements for takeoff and landings, anddecreased rate of climb. An average small airplane,for example, requiring 1,000 feet for takeoff at sealevel under standard atmospheric conditions willrequire a takeoff run of approximately 2,000 feet at anoperational altitude of 5,000 feet.

NOTE−A turbo-charged aircraft engine provides some slightadvantage in that it provides sea level horsepower up to aspecified altitude above sea level.

1. Density Altitude Advisories. At airportswith elevations of 2,000 feet and higher, controltowers and FSSs will broadcast the advisory “CheckDensity Altitude” when the temperature reaches apredetermined level. These advisories will bebroadcast on appropriate tower frequencies or, whereavailable, ATIS. FSSs will broadcast these advisoriesas a part of Local Airport Advisory, and on TWEB.

2. These advisories are provided by air trafficfacilities, as a reminder to pilots that hightemperatures and high field elevations will causesignificant changes in aircraft characteristics. Thepilot retains the responsibility to compute densityaltitude, when appropriate, as a part of preflightduties.

NOTE−All FSSs will compute the current density altitude uponrequest.

j. Mountain Wave. Many pilots go all their liveswithout understanding what a mountain wave is.Quite a few have lost their lives because of this lackof understanding. One need not be a licensedmeteorologist to understand the mountain wavephenomenon.

AIM8/15/19


1. Mountain waves occur when air is beingblown over a mountain range or even the ridge of asharp bluff area. As the air hits the upwind side of therange, it starts to climb, thus creating what isgenerally a smooth updraft which turns into aturbulent downdraft as the air passes the crest of theridge. From this point, for many miles downwind,there will be a series of downdrafts and updrafts.Satellite photos of the Rockies have shown mountainwaves extending as far as 700 miles downwind of therange. Along the east coast area, such photos of theAppalachian chain have picked up the mountainwave phenomenon over a hundred miles eastward.All it takes to form a mountain wave is wind blowingacross the range at 15 knots or better at an intersectionangle of not less than 30 degrees.

2. Pilots from flatland areas should understanda few things about mountain waves in order to stayout of trouble. When approaching a mountain rangefrom the upwind side (generally the west), there willusually be a smooth updraft; therefore, it is not quiteas dangerous an area as the lee of the range. From theleeward side, it is always a good idea to add an extrathousand feet or so of altitude because downdraftscan exceed the climb capability of the aircraft. Neverexpect an updraft when approaching a mountainchain from the leeward. Always be prepared to copewith a downdraft and turbulence.

3. When approaching a mountain ridge from thedownwind side, it is recommended that the ridge beapproached at approximately a 45 degree angle to thehorizontal direction of the ridge. This permits a saferretreat from the ridge with less stress on the aircraftshould severe turbulence and downdraft be experi-enced. If severe turbulence is encountered,simultaneously reduce power and adjust pitch untilaircraft approaches maneuvering speed, then adjustpower and trim to maintain maneuvering speed andfly away from the turbulent area.

7−5−7. Use of Runway Half−way Signs atUnimproved Airports

When installed, runway half−way signs provide thepilot with a reference point to judge takeoffacceleration trends. Assuming that the runway lengthis appropriate for takeoff (considering runway

condition and slope, elevation, aircraft weight, wind,and temperature), typical takeoff acceleration shouldallow the airplane to reach 70 percent of lift−offairspeed by the midpoint of the runway. The “rule ofthumb” is that should airplane acceleration not allowthe airspeed to reach this value by the midpoint, thetakeoff should be aborted, as it may not be possible toliftoff in the remaining runway.

Several points are important when considering usingthis “rule of thumb”:

a. Airspeed indicators in small airplanes are notrequired to be evaluated at speeds below stalling, andmay not be usable at 70 percent of liftoff airspeed.

b. This “rule of thumb” is based on a uniformsurface condition. Puddles, soft spots, areas of talland/or wet grass, loose gravel, etc., may impedeacceleration or even cause deceleration. Even if theairplane achieves 70 percent of liftoff airspeed by themidpoint, the condition of the remainder of the runwaymay not allow further acceleration. The entire lengthof the runway should be inspected prior to takeoff toensure a usable surface.

c. This “rule of thumb” applies only to runwayrequired for actual liftoff. In the event that obstaclesaffect the takeoff climb path, appropriate distancemust be available after liftoff to accelerate to best angleof climb speed and to clear the obstacles. This will, ineffect, require the airplane to accelerate to a higherspeed by midpoint, particularly if the obstacles areclose to the end of the runway. In addition, thistechnique does not take into account the effects ofupslope or tailwinds on takeoff performance. Thesefactors will also require greater acceleration thannormal and, under some circumstances, preventtakeoff entirely.

d. Use of this “rule of thumb” does not alleviate thepilot’s responsibility to comply with applicableFederal Aviation Regulations, the limitations andperformance data provided in the FAA approvedAirplane Flight Manual (AFM), or, in the absence ofan FAA approved AFM, other data provided by theaircraft manufacturer.

In addition to their use during takeoff, runwayhalf−way signs offer the pilot increased awareness ofhis or her position along the runway during landingoperations.

AIM 8/15/19


NOTE−No FAA standard exists for the appearance of the runwayhalf−way sign. FIG 7−5−1 shows a graphical depiction ofa typical runway half−way sign.

7−5−8. Seaplane Safety

a. Acquiring a seaplane class rating affords accessto many areas not available to landplane pilots.Adding a seaplane class rating to your pilot certificatecan be relatively uncomplicated and inexpensive.However, more effort is required to become a safe,efficient, competent “bush” pilot. The natural hazardsof the backwoods have given way to modernman-made hazards. Except for the far north, theavailable bodies of water are no longer the exclusivedomain of the airman. Seaplane pilots must bevigilant for hazards such as electric power lines,power, sail and rowboats, rafts, mooring lines, waterskiers, swimmers, etc.

FIG 7−5−1

Typical Runway Half−way Sign

b. Seaplane pilots must have a thorough under-standing of the right-of-way rules as they apply toaircraft versus other vessels. Seaplane pilots areexpected to know and adhere to both the U.S. CoastGuard’s (USCG) Navigation Rules, International−In-land, and 14 CFR Section 91.115, Right−of−WayRules; Water Operations. The navigation rules of theroad are a set of collision avoidance rules as theyapply to aircraft on the water. A seaplane isconsidered a vessel when on the water for thepurposes of these collision avoidance rules. Ingeneral, a seaplane on the water must keep well clear

of all vessels and avoid impeding their navigation.The CFR requires, in part, that aircraft operating onthe water “. . . shall, insofar as possible, keep clear ofall vessels and avoid impeding their navigation, andshall give way to any vessel or other aircraft that isgiven the right−of−way . . . .” This means that aseaplane should avoid boats and commercialshipping when on the water. If on a collision course,the seaplane should slow, stop, or maneuver to theright, away from the bow of the oncoming vessel.Also, while on the surface with an engine running, anaircraft must give way to all nonpowered vessels.Since a seaplane in the water may not be asmaneuverable as one in the air, the aircraft on thewater has right-of-way over one in the air, and onetaking off has right-of-way over one landing. Aseaplane is exempt from the USCG safety equipmentrequirements, including the requirements for Person-al Flotation Devices (PFD). Requiring seaplanes onthe water to comply with USCG equipmentrequirements in addition to the FAA equipmentrequirements would be an unnecessary burden onseaplane owners and operators.

c. Unless they are under Federal jurisdiction,navigable bodies of water are under the jurisdictionof the state, or in a few cases, privately owned. Unlessthey are specifically restricted, aircraft have as muchright to operate on these bodies of water as othervessels. To avoid problems, check with Federal orlocal officials in advance of operating on unfamiliarwaters. In addition to the agencies listed inTBL 7−5−1, the nearest Flight Standards DistrictOffice can usually offer some practical suggestions aswell as regulatory information. If you land on arestricted body of water because of an inflightemergency, or in ignorance of the restrictions youhave violated, report as quickly as practical to thenearest local official having jurisdiction and explainyour situation.

d. When operating a seaplane over or into remoteareas, appropriate attention should be given tosurvival gear. Minimum kits are recommended forsummer and winter, and are required by law for flightinto sparsely settled areas of Canada and Alaska.Alaska State Department of Transportation andCanadian Ministry of Transport officials can providespecific information on survival gear requirements.The kit should be assembled in one container and beeasily reachable and preferably floatable.

AIM8/15/19


TBL 7−5−1

Jurisdictions Controlling Navigable Bodies of Water

Authority to Consult For Use of a Body of Water

Location Authority Contact

Wilderness Area U.S. Departmentof Agriculture,Forest Service

Local forest ranger

National Forest USDA ForestService

Local forest ranger

National Park U.S. Departmentof the Interior,National ParkService

Local park ranger

Indian Reservation USDI, Bureau ofIndian Affairs

Local Bureauoffice

State Park State governmentor state forestry orpark service

Local stateaviation office forfurtherinformation

Canadian Nationaland ProvincialParks

Supervised andrestricted on anindividual basisfrom province toprovince and bydifferentdepartments of theCanadiangovernment;consult CanadianFlight InformationManual and/orWater AerodromeSupplement

ParkSuperintendent inan emergency

e. The FAA recommends that each seaplane owneror operator provide flotation gear for occupants anytime a seaplane operates on or near water. 14 CFRSection 91.205(b)(12) requires approved flotationgear for aircraft operated for hire over water andbeyond power-off gliding distance from shore.FAA-approved gear differs from that required fornavigable waterways under USCG rules. FAA-ap-proved life vests are inflatable designs as comparedto the USCG’s noninflatable PFD’s that may consistof solid, bulky material. Such USCG PFDs areimpractical for seaplanes and other aircraft becausethey may block passage through the relatively narrowexits available to pilots and passengers. Life vestsapproved under Technical Standard Order (TSO)TSO−C13E contain fully inflatable compartments.The wearer inflates the compartments (AFTERexiting the aircraft) primarily by independent CO2cartridges, with an oral inflation tube as a backup. Theflotation gear also contains a water-activated,self-illuminating signal light. The fact that pilots and

passengers can easily don and wear inflatable lifevests (when not inflated) provides maximumeffectiveness and allows for unrestricted movement.It is imperative that passengers are briefed on thelocation and proper use of available PFDs prior toleaving the dock.

f. The FAA recommends that seaplane owners andoperators obtain Advisory Circular (AC) 91−69,Seaplane Safety for 14 CFR Part 91 Operations, freefrom the U.S. Department of Transportation,Subsequent Distribution Office, SVC−121.23, Ard-more East Business Center, 3341 Q 75th Avenue,Landover, MD 20785; fax: (301) 386−5394. TheUSCG Navigation Rules International−Inland(COMDTINSTM 16672.2B) is available for a feefrom the Government Publishing Office by facsimilerequest to (202) 512−2250, and can be ordered usingMastercard or Visa.

7−5−9. Flight Operations in Volcanic Ash

a. Severe volcanic eruptions which send ash andsulphur dioxide (SO2) gas into the upper atmosphereoccur somewhere around the world several timeseach year. Flying into a volcanic ash cloud can beexceedingly dangerous. A B747−200 lost all fourengines after such an encounter and a B747−400 hadthe same nearly catastrophic experience. Piston−powered aircraft are less likely to lose power butsevere damage is almost certain to ensue after anencounter with a volcanic ash cloud which is only afew hours old.

b. Most important is to avoid any encounter withvolcanic ash. The ash plume may not be visible,especially in instrument conditions or at night; andeven if visible, it is difficult to distinguish visuallybetween an ash cloud and an ordinary weather cloud.Volcanic ash clouds are not displayed on airborne orATC radar. The pilot must rely on reports from airtraffic controllers and other pilots to determine thelocation of the ash cloud and use that information toremain well clear of the area. Additionally, thepresence of a sulphur-like odor throughout the cabinmay indicate the presence of SO2 emitted by volcanicactivity, but may or may not indicate the presence ofvolcanic ash. Every attempt should be made to remainon the upwind side of the volcano.

c. It is recommended that pilots encountering anash cloud should immediately reduce thrust to idle(altitude permitting), and reverse course in order to

AIM 8/15/19


escape from the cloud. Ash clouds may extend forhundreds of miles and pilots should not attempt to flythrough or climb out of the cloud. In addition, thefollowing procedures are recommended:

1. Disengage the autothrottle if engaged. Thiswill prevent the autothrottle from increasing enginethrust;

2. Turn on continuous ignition;

3. Turn on all accessory airbleeds including allair conditioning packs, nacelles, and wing anti-ice.This will provide an additional engine stall margin byreducing engine pressure.

d. The following has been reported by flightcrewswho have experienced encounters with volcanic dustclouds:

1. Smoke or dust appearing in the cockpit.

2. An acrid odor similar to electrical smoke.

3. Multiple engine malfunctions, such ascompressor stalls, increasing EGT, torching fromtailpipe, and flameouts.

4. At night, St. Elmo’s fire or other staticdischarges accompanied by a bright orange glow inthe engine inlets.

5. A fire warning in the forward cargo area.

e. It may become necessary to shut down and thenrestart engines to prevent exceeding EGT limits.Volcanic ash may block the pitot system and result inunreliable airspeed indications.

f. If you see a volcanic eruption and have not beenpreviously notified of it, you may have been the firstperson to observe it. In this case, immediately contactATC and alert them to the existence of the eruption.If possible, use the Volcanic Activity Reporting form(VAR) depicted in Appendix 2 of this manual.Items 1 through 8 of the VAR should be transmittedimmediately. The information requested initems 9 through 16 should be passed after landing. Ifa VAR form is not immediately available, relayenough information to identify the position andnature of the volcanic activity. Do not becomeunnecessarily alarmed if there is merely steam or verylow-level eruptions of ash.

g. When landing at airports where volcanic ash hasbeen deposited on the runway, be aware that even athin layer of dry ash can be detrimental to braking

action. Wet ash on the runway may also reduceeffectiveness of braking. It is recommended thatreverse thrust be limited to minimum practical toreduce the possibility of reduced visibility and engineingestion of airborne ash.

h. When departing from airports where volcanicash has been deposited, it is recommended that pilotsavoid operating in visible airborne ash. Allow ash tosettle before initiating takeoff roll. It is alsorecommended that flap extension be delayed untilinitiating the before takeoff checklist and that arolling takeoff be executed to avoid blowing ash backinto the air.

7−5−10. Emergency Airborne Inspection ofOther Aircraft

a. Providing airborne assistance to another aircraftmay involve flying in very close proximity to thataircraft. Most pilots receive little, if any, formaltraining or instruction in this type of flying activity.Close proximity flying without sufficient time to plan(i.e., in an emergency situation), coupled with thestress involved in a perceived emergency can behazardous.

b. The pilot in the best position to assess thesituation should take the responsibility of coordinat-ing the airborne intercept and inspection, and takeinto account the unique flight characteristics anddifferences of the category(s) of aircraft involved.

c. Some of the safety considerations are:

1. Area, direction and speed of the intercept;

2. Aerodynamic effects (i.e., rotorcraft down-wash);

3. Minimum safe separation distances;

4. Communications requirements, lost commu-nications procedures, coordination with ATC;

5. Suitability of diverting the distressed aircraftto the nearest safe airport; and

6. Emergency actions to terminate the intercept.

d. Close proximity, inflight inspection of anotheraircraft is uniquely hazardous. The pilot−in−command of the aircraft experiencing theproblem/emergency must not relinquish control ofthe situation and/or jeopardize the safety of theiraircraft. The maneuver must be accomplished withminimum risk to both aircraft.

AIM8/15/19


7−5−11. Precipitation Static

a. Precipitation static is caused by aircraft in flightcoming in contact with uncharged particles. Theseparticles can be rain, snow, fog, sleet, hail, volcanicash, dust; any solid or liquid particles. When theaircraft strikes these neutral particles the positiveelement of the particle is reflected away from theaircraft and the negative particle adheres to the skinof the aircraft. In a very short period of time asubstantial negative charge will develop on the skinof the aircraft. If the aircraft is not equipped withstatic dischargers, or has an ineffective staticdischarger system, when a sufficient negative voltagelevel is reached, the aircraft may go into“CORONA.” That is, it will discharge the staticelectricity from the extremities of the aircraft, such asthe wing tips, horizontal stabilizer, vertical stabilizer,antenna, propeller tips, etc. This discharge of staticelectricity is what you will hear in your headphonesand is what we call P−static.

b. A review of pilot reports often shows differentsymptoms with each problem that is encountered.The following list of problems is a summary of manypilot reports from many different aircraft. Eachproblem was caused by P−static:

1. Complete loss of VHF communications.

2. Erroneous magnetic compass readings(30 percent in error).

3. High pitched squeal on audio.

4. Motor boat sound on audio.

5. Loss of all avionics in clouds.

6. VLF navigation system inoperative most ofthe time.

7. Erratic instrument readouts.

8. Weak transmissions and poor receptivity ofradios.

9. “St. Elmo’s Fire” on windshield.

c. Each of these symptoms is caused by onegeneral problem on the airframe. This problem is theinability of the accumulated charge to flow easily tothe wing tips and tail of the airframe, and properlydischarge to the airstream.

d. Static dischargers work on the principal ofcreating a relatively easy path for dischargingnegative charges that develop on the aircraft by usinga discharger with fine metal points, carbon coatedrods, or carbon wicks rather than wait until a largecharge is developed and discharged off the trailingedges of the aircraft that will interfere with avionicsequipment. This process offers approximately50 decibels (dB) static noise reduction which isadequate in most cases to be below the threshold ofnoise that would cause interference in avionicsequipment.

e. It is important to remember that precipitationstatic problems can only be corrected with the propernumber of quality static dischargers, properlyinstalled on a properly bonded aircraft. P−static isindeed a problem in the all weather operation of theaircraft, but there are effective ways to combat it. Allpossible methods of reducing the effects of P−staticshould be considered so as to provide the bestpossible performance in the flight environment.

f. A wide variety of discharger designs is availableon the commercial market. The inclusion ofwell−designed dischargers may be expected toimprove airframe noise in P−static conditions by asmuch as 50 dB. Essentially, the discharger providesa path by which accumulated charge may leave theairframe quietly. This is generally accomplished byproviding a group of tiny corona points to permitonset of corona−current flow at a low aircraftpotential. Additionally, aerodynamic design ofdischargers to permit corona to occur at the lowestpossible atmospheric pressure also lowers the coronathreshold. In addition to permitting a low−potentialdischarge, the discharger will minimize the radiationof radio frequency (RF) energy which accompaniesthe corona discharge, in order to minimize effects ofRF components at communications and navigationfrequencies on avionics performance. These effectsare reduced through resistive attachment of thecorona point(s) to the airframe, preserving directcurrent connection but attenuating the higher−fre-quency components of the discharge.

g. Each manufacturer of static dischargers offersinformation concerning appropriate discharger loca-tion on specific airframes. Such locations emphasizethe trailing outboard surfaces of wings and horizontaltail surfaces, plus the tip of the vertical stabilizer,where charge tends to accumulate on the airframe.

AIM 8/15/19


Sufficient dischargers must be provided to allow forcurrent−carrying capacity which will maintainairframe potential below the corona threshold of thetrailing edges.

h. In order to achieve full performance of avionicequipment, the static discharge system will requireperiodic maintenance. A pilot knowledgeable ofP−static causes and effects is an important element inassuring optimum performance by early recognitionof these types of problems.

7−5−12. Light Amplification by StimulatedEmission of Radiation (Laser) Operationsand Reporting Illumination of Aircraft

a. Lasers have many applications. Of concern tousers of the National Airspace System are those laserevents that may affect pilots, e.g., outdoor laser lightshows or demonstrations for entertainment andadvertisements at special events and theme parks.Generally, the beams from these events appear asbright blue−green in color; however, they may be red,yellow, or white. However, some laser systemsproduce light which is invisible to the human eye.

b. FAA regulations prohibit the disruption ofaviation activity by any person on the ground or in theair. The FAA and the Food and Drug Administration(the Federal agency that has the responsibility toenforce compliance with Federal requirements forlaser systems and laser light show products) areworking together to ensure that operators of thesedevices do not pose a hazard to aircraft operators.

c. Pilots should be aware that illumination fromthese laser operations are able to create temporaryvision impairment miles from the actual location. Inaddition, these operations can produce permanent eyedamage. Pilots should make themselves aware ofwhere these activities are being conducted and avoidthese areas if possible.

d. Recent and increasing incidents of unautho-rized illumination of aircraft by lasers, as well as theproliferation and increasing sophistication of laserdevices available to the general public, dictates thatthe FAA, in coordination with other governmentagencies, take action to safeguard flights from theseunauthorized illuminations.

e. Pilots should report laser illumination activity tothe controlling Air Traffic Control facilities, FederalContract Towers or Flight Service Stations as soon aspossible after the event. The following informationshould be included:

1. UTC Date and Time of Event.

2. Call Sign or Aircraft Registration Number.

3. Type Aircraft.

4. Nearest Major City.

5. Altitude.

6. Location of Event (Latitude/Longitude and/or Fixed Radial Distance (FRD)).

7. Brief Description of the Event and any otherPertinent Information.

f. Pilots are also encouraged to complete theLaser Beam Exposure Questionnaire locatedon the FAA Laser Safety Initiative website athttp://www.faa.gov/about/initiatives/lasers/and submit electronically per the directions on thequestionnaire, as soon as possible after landing.

g. When a laser event is reported to an air trafficfacility, a general caution warning will be broad-casted on all appropriate frequencies everyfive minutes for 20 minutes and broadcasted on theATIS for one hour following the report.

PHRASEOLOGY−UNAUTHORIZED LASER ILLUMINATION EVENT,(UTC time), (location), (altitude), (color), (direction).

EXAMPLE−“Unauthorized laser illumination event, at 0100z, 8 milefinal runway 18R at 3,000 feet, green laser from thesouthwest.”

REFERENCE−FAA Order JO 7110.65, Paragraph 10−2−14, Unauthorized LaserIllumination of AircraftFAA Order JO 7210.3, Paragraph 2−1−27, Reporting UnauthorizedLaser Illumination of Aircraft

h. When these activities become known to theFAA, Notices to Airmen (NOTAMs) are issued toinform the aviation community of the events. Pilotsshould consult NOTAMs or the Special Noticessection of the Chart Supplement U.S. for informationregarding these activities.

AIM8/15/19


7−5−13. Flying in Flat Light, Brown OutConditions, and White Out Conditions

a. Flat Light. Flat light is an optical illusion, alsoknown as “sector or partial white out.” It is not assevere as “white out” but the condition causes pilotsto lose their depth−of−field and contrast in vision.Flat light conditions are usually accompanied byovercast skies inhibiting any visual clues. Suchconditions can occur anywhere in the world,primarily in snow covered areas but can occur in dust,sand, mud flats, or on glassy water. Flat light cancompletely obscure features of the terrain, creating aninability to distinguish distances and closure rates.As a result of this reflected light, it can give pilots theillusion that they are ascending or descending whenthey may actually be flying level. However, withgood judgment and proper training and planning, it ispossible to safely operate an aircraft in flat lightconditions.

b. Brown Out. A brownout (or brown−out) is anin−flight visibility restriction due to dust or sand inthe air. In a brownout, the pilot cannot see nearbyobjects which provide the outside visual referencesnecessary to control the aircraft near the ground. Thiscan cause spatial disorientation and loss of situationalawareness leading to an accident.

1. The following factors will affect theprobability and severity of brownout: rotor diskloading, rotor configuration, soil composition, wind,approach speed, and approach angle.

2. The brownout phenomenon causes accidentsduring helicopter landing and take−off operations indust, fine dirt, sand, or arid desert terrain. Intense,blinding dust clouds stirred up by the helicopter rotordownwash during near−ground flight causes signifi-cant flight safety risks from aircraft and groundobstacle collisions, and dynamic rollover due tosloped and uneven terrain.

3. This is a dangerous phenomenon experiencedby many helicopters when making landing approach-es in dusty environments, whereby sand or dustparticles become swept up in the rotor outwash andobscure the pilot’s vision of the terrain. This isparticularly dangerous because the pilot needs thosevisual cues from their surroundings in order to makea safe landing.

4. Blowing sand and dust can cause an illusionof a tilted horizon. A pilot not using the flight

instruments for reference may instinctively try tolevel the aircraft with respect to the false horizon,resulting in an accident. Helicopter rotor wash alsocauses sand to blow around outside the cockpitwindows, possibly leading the pilot to experience anillusion where the helicopter appears to be turningwhen it is actually in a level hover. This can also causethe pilot to make incorrect control inputs which canquickly lead to disaster when hovering near theground. In night landings, aircraft lighting canenhance the visual illusions by illuminating thebrownout cloud.

c. White Out. As defined in meteorologicalterms, white out occurs when a person becomesengulfed in a uniformly white glow. The glow is aresult of being surrounded by blowing snow, dust,sand, mud or water. There are no shadows, no horizonor clouds and all depth−of−field and orientation arelost. A white out situation is severe in that there areno visual references. Flying is not recommended inany white out situation. Flat light conditions can leadto a white out environment quite rapidly, and bothatmospheric conditions are insidious; they sneak upon you as your visual references slowly begin todisappear. White out has been the cause of severalaviation accidents.

d. Self Induced White Out. This effect typicallyoccurs when a helicopter takes off or lands on asnow−covered area. The rotor down wash picks upparticles and re−circulates them through the rotordown wash. The effect can vary in intensitydepending upon the amount of light on the surface.This can happen on the sunniest, brightest day withgood contrast everywhere. However, when ithappens, there can be a complete loss of visual clues.If the pilot has not prepared for this immediate loss ofvisibility, the results can be disastrous. Goodplanning does not prevent one from encountering flatlight or white out conditions.

e. Never take off in a white out situation.

1. Realize that in flat light conditions it may bepossible to depart but not to return to that site. Duringtakeoff, make sure you have a reference point. Do notlose sight of it until you have a departure referencepoint in view. Be prepared to return to the takeoffreference if the departure reference does not comeinto view.

2. Flat light is common to snow skiers. One wayto compensate for the lack of visual contrast and

AIM 8/15/19


depth−of−field loss is by wearing amber tinted lenses(also known as blue blockers). Special note ofcaution: Eyewear is not ideal for every pilot. Takeinto consideration personal factors − age, lightsensitivity, and ambient lighting conditions.

3. So what should a pilot do when all visualreferences are lost?

(a) Trust the cockpit instruments.

(b) Execute a 180 degree turnaround and startlooking for outside references.

(c) Above all − fly the aircraft.

f. Landing in Low Light Conditions. Whenlanding in a low light condition − use extremecaution. Look for intermediate reference points, inaddition to checkpoints along each leg of the route forcourse confirmation and timing. The lower theambient light becomes, the more reference points apilot should use.

g. Airport Landings.

1. Look for features around the airport orapproach path that can be used in determining depthperception. Buildings, towers, vehicles or otheraircraft serve well for this measurement. Usesomething that will provide you with a sense of heightabove the ground, in addition to orienting you to therunway.

2. Be cautious of snowdrifts and snow banks −anything that can distinguish the edge of the runway.Look for subtle changes in snow texture or shading toidentify ridges or changes in snow depth.

h. Off−Airport Landings.

1. In the event of an off−airport landing, pilotshave used a number of different visual cues to gainreference. Use whatever you must to create thecontrast you need. Natural references seem to workbest (trees, rocks, snow ribs, etc.)

(a) Over flight.

(b) Use of markers.

(c) Weighted flags.

(d) Smoke bombs.

(e) Any colored rags.

(f) Dye markers.

(g) Kool−aid.

(h) Trees or tree branches.

2. It is difficult to determine the depth of snowin areas that are level. Dropping items from theaircraft to use as reference points should be used as avisual aid only and not as a primary landing reference.Unless your marker is biodegradable, be sure toretrieve it after landing. Never put yourself in aposition where no visual references exist.

3. Abort landing if blowing snow obscures yourreference. Make your decisions early. Don’t assumeyou can pick up a lost reference point when you getcloser.

4. Exercise extreme caution when flying fromsunlight into shade. Physical awareness may tell youthat you are flying straight but you may actually be ina spiral dive with centrifugal force pressing againstyou. Having no visual references enhances thisillusion. Just because you have a good visualreference does not mean that it’s safe to continue.There may be snow−covered terrain not visible in thedirection that you are traveling. Getting caught in a novisual reference situation can be fatal.

i. Flying Around a Lake.

1. When flying along lakeshores, use them as areference point. Even if you can see the other side,realize that your depth perception may be poor. It iseasy to fly into the surface. If you must cross the lake,check the altimeter frequently and maintain a safealtitude while you still have a good reference. Don’tdescend below that altitude.

2. The same rules apply to seemingly flat areasof snow. If you don’t have good references, avoidgoing there.

j. Other Traffic. Be on the look out for othertraffic in the area. Other aircraft may be using yoursame reference point. Chances are greater ofcolliding with someone traveling in the samedirection as you, than someone flying in the oppositedirection.

k. Ceilings. Low ceilings have caught manypilots off guard. Clouds do not always form parallelto the surface, or at the same altitude. Pilots may tryto compensate for this by flying with a slight bank andthus creating a descending turn.

AIM8/15/19


l. Glaciers. Be conscious of your altitude whenflying over glaciers. The glaciers may be rising fasterthan you are climbing.

7−5−14. Operations in Ground IcingConditions

a. The presence of aircraft airframe icing duringtakeoff, typically caused by improper or no deicing ofthe aircraft being accomplished prior to flight hascontributed to many recent accidents in turbineaircraft. The General Aviation Joint SteeringCommittee (GAJSC) is the primary vehicle forgovernment−industry cooperation, communication,and coordination on GA accident mitigation. TheTurbine Aircraft Operations Subgroup (TAOS)works to mitigate accidents in turbine accidentaviation. While there is sufficient information andguidance currently available regarding the effects oficing on aircraft and methods for deicing, the TAOShas developed a list of recommended actions tofurther assist pilots and operators in this area.

While the efforts of the TAOS specifically focus onturbine aircraft, it is recognized that their recommen-dations are applicable to and can be adapted for thepilot of a small, piston powered aircraft too.

b. The following recommendations are offered:

1. Ensure that your aircraft’s lift−generatingsurfaces are COMPLETELY free of contaminationbefore flight through a tactile (hands on) check of thecritical surfaces when feasible. Even when otherwisepermitted, operators should avoid smooth or polishedfrost on lift−generating surfaces as an acceptablepreflight condition.

2. Review and refresh your cold weatherstandard operating procedures.

3. Review and be familiar with the AirplaneFlight Manual (AFM) limitations and proceduresnecessary to deal with icing conditions prior to flight,as well as in flight.

4. Protect your aircraft while on the ground, ifpossible, from sleet and freezing rain by takingadvantage of aircraft hangars.

5. Take full advantage of the opportunitiesavailable at airports for deicing. Do not refuse deicingservices simply because of cost.

6. Always consider canceling or delaying aflight if weather conditions do not support a safeoperation.

c. If you haven’t already developed a set ofStandard Operating Procedures for cold weatheroperations, they should include:

1. Procedures based on information that isapplicable to the aircraft operated, such as AFMlimitations and procedures;

2. Concise and easy to understand guidance thatoutlines best operational practices;

3. A systematic procedure for recognizing,evaluating and addressing the associated icing risk,and offer clear guidance to mitigate this risk;

4. An aid (such as a checklist or reference cards)that is readily available during normal day−to−dayaircraft operations.

d. There are several sources for guidance relatingto airframe icing, including:

1. http://aircrafticing.grc.nasa.gov/index.html

2. http://www.ibac.org/is−bao/isbao.htm

3. http://www.natasafety1st.org/bus_deice.htm

4. Advisory Circular (AC) 91−74, Pilot Guide,Flight in Icing Conditions.

5. AC 135−17, Pilot Guide Small AircraftGround Deicing.

6. AC 135−9, FAR Part 135 Icing Limitations.

7. AC 120−60, Ground Deicing and Anti−icingProgram.

8. AC 135−16, Ground Deicing and Anti−icingTraining and Checking.

The FAA Approved Deicing Program Updates ispublished annually as a Flight Standards InformationBulletin for Air Transportation and contains detailedinformation on deicing and anti−icing procedures andholdover times. It may be accessed at the followingwebsite by selecting the current year’s informationbulletins:http://www.faa.gov/library/manuals/examiners_inspectors/8400/fsat

AIM 8/15/19


7−5−15. Avoid Flight in the Vicinity ofExhaust Plumes (Smoke Stacks andCooling Towers)

a. Flight Hazards Exist Around ExhaustPlumes. Exhaust plumes are defined as visible orinvisible emissions from power plants, industrialproduction facilities, or other industrial systems thatrelease large amounts of vertically directed unstablegases (effluent). High temperature exhaust plumescan cause significant air disturbances such asturbulence and vertical shear. Other identifiedpotential hazards include, but are not necessarilylimited to: reduced visibility, oxygen depletion,engine particulate contamination, exposure togaseous oxides, and/or icing. Results of encounteringa plume may include airframe damage, aircraft upset,and/or engine damage/failure. These hazards aremost critical during low altitude flight in calm andcold air, especially in and around approach anddeparture corridors or airport traffic areas. Whether plumes are visible or invisible, the totalextent of their turbulent affect is difficult to predict.Some studies do predict that the significant turbulenteffects of an exhaust plume can extend to heights ofover 1,000 feet above the height of the top of the stackor cooling tower. Any effects will be morepronounced in calm stable air where the plume is veryhot and the surrounding area is still and cold.Fortunately, studies also predict that any amount ofcrosswind will help to dissipate the effects. However,the size of the tower or stack is not a good indicatorof the predicted effect the plume may produce. Themajor effects are related to the heat or size of the

plume effluent, the ambient air temperature, and thewind speed affecting the plume. Smaller aircraft canexpect to feel an effect at a higher altitude thanheavier aircraft.

b. When able, a pilot should steer clear ofexhaust plumes by flying on the upwind side ofsmokestacks or cooling towers. When a plume isvisible via smoke or a condensation cloud, remainclear and realize a plume may have both visible andinvisible characteristics. Exhaust stacks withoutvisible plumes may still be in full operation, andairspace in the vicinity should be treated with caution.As with mountain wave turbulence or clear airturbulence, an invisible plume may be encounteredunexpectedly. Cooling towers, power plant stacks,exhaust fans, and other similar structures are depictedin FIG 7−5−2.

Pilots are encouraged to exercise caution when flyingin the vicinity of exhaust plumes. Pilots are alsoencouraged to reference the Chart Supplement U.S.where amplifying notes may caution pilots andidentify the location of structure(s) emitting exhaustplumes.

The best available information on this phenomenonmust come from pilots via the PIREP reportingprocedures. All pilots encountering hazardousplume conditions are urgently requested to reporttime, location, and intensity (light, moderate, severe,or extreme) of the element to the FAA facility withwhich they are maintaining radio contact. If time andconditions permit, elements should be reportedaccording to the standards for other PIREPs andposition reports (AIM Paragraph 7−1−23, PIREPSRelating to Turbulence).

FIG 7−5−2

Plumes

AIM10/12/17

7−6−1Safety, Accident, and Hazard Reports

Section 6. Safety, Accident, and Hazard Reports

7−6−1. Aviation Safety Reporting Program

a. The FAA has established a voluntary AviationSafety Reporting Program designed to stimulate thefree and unrestricted flow of information concerningdeficiencies and discrepancies in the aviation system.This is a positive program intended to ensure thesafest possible system by identifying and correctingunsafe conditions before they lead to accidents. Theprimary objective of the program is to obtaininformation to evaluate and enhance the safety andefficiency of the present system.

b. This cooperative safety reporting programinvites pilots, controllers, flight attendants, mainte-nance personnel and other users of the airspacesystem, or any other person, to file written reports ofactual or potential discrepancies and deficienciesinvolving the safety of aviation operations. Theoperations covered by the program include departure,en route, approach, and landing operations andprocedures, air traffic control procedures andequipment, crew and air traffic control communica-tions, aircraft cabin operations, aircraft movement onthe airport, near midair collisions, aircraft mainte-nance and record keeping and airport conditions orservices.

c. The report should give the date, time, location,persons and aircraft involved (if applicable), natureof the event, and all pertinent details.

d. To ensure receipt of this information, theprogram provides for the waiver of certaindisciplinary actions against persons, including pilotsand air traffic controllers, who file timely writtenreports concerning potentially unsafe incidents. To beconsidered timely, reports must be delivered orpostmarked within 10 days of the incident unless thatperiod is extended for good cause. Reports should besubmitted on NASA ARC Forms 277, which areavailable free of charge, postage prepaid, at FAAFlight Standards District Offices and Flight ServiceStations, and from NASA, ASRS, PO Box 189,Moffet Field, CA 94035.

e. The FAA utilizes the National Aeronautics andSpace Administration (NASA) to act as anindependent third party to receive and analyze reportssubmitted under the program. This program is

described in AC 00−46, Aviation Safety ReportingProgram.

7−6−2. Aircraft Accident and IncidentReporting

a. Occurrences Requiring Notification. Theoperator of an aircraft must immediately, and by themost expeditious means available, notify the nearestNational Transportation Safety Board (NTSB) FieldOffice when:

1. An aircraft accident or any of the followinglisted incidents occur:

(a) Flight control system malfunction orfailure.

(b) Inability of any required flight crewmember to perform their normal flight duties as aresult of injury or illness.

(c) Failure of structural components of aturbine engine excluding compressor and turbineblades and vanes.

(d) Inflight fire.

(e) Aircraft collide in flight.

(f) Damage to property, other than theaircraft, estimated to exceed $25,000 for repair(including materials and labor) or fair market value inthe event of total loss, whichever is less.

(g) For large multi-engine aircraft (more than12,500 pounds maximum certificated takeoffweight):

(1) Inflight failure of electrical systemswhich requires the sustained use of an emergency buspowered by a back-up source such as a battery,auxiliary power unit, or air-driven generator to retainflight control or essential instruments;

(2) Inflight failure of hydraulic systemsthat results in sustained reliance on the sole remaininghydraulic or mechanical system for movement offlight control surfaces;

(3) Sustained loss of the power or thrustproduced by two or more engines; and

(4) An evacuation of aircraft in which anemergency egress system is utilized.

AIM 10/12/17

7−6−2 Safety, Accident, and Hazard Reports

2. An aircraft is overdue and is believed to havebeen involved in an accident.

b. Manner of Notification.

1. The most expeditious method of notificationto the NTSB by the operator will be determined by thecircumstances existing at that time. The NTSB hasadvised that any of the following would beconsidered examples of the type of notification thatwould be acceptable:

(a) Direct telephone notification.

(b) Telegraphic notification.

(c) Notification to the FAA who would in turnnotify the NTSB by direct communication; i.e., dis-patch or telephone.

c. Items to be Included in Notification. Thenotification required above must contain thefollowing information, if available:

1. Type, nationality, and registration marks ofthe aircraft.

2. Name of owner and operator of the aircraft.

3. Name of the pilot-in-command.

4. Date and time of the accident, or incident.

5. Last point of departure, and point of intendedlanding of the aircraft.

6. Position of the aircraft with reference to someeasily defined geographical point.

7. Number of persons aboard, number killed,and number seriously injured.

8. Nature of the accident, or incident, theweather, and the extent of damage to the aircraft so faras is known; and

9. A description of any explosives, radioactivematerials, or other dangerous articles carried.

d. Follow−up Reports.

1. The operator must file a report on NTSBForm 6120.1 or 6120.2, available from NTSB FieldOffices or from the NTSB, Washington, DC, 20594:

(a) Within 10 days after an accident;

(b) When, after 7 days, an overdue aircraft isstill missing;

(c) A report on an incident for whichnotification is required as described in subpara-graph a(1) must be filed only as requested by anauthorized representative of the NTSB.

2. Each crewmember, if physically able at thetime the report is submitted, must attach a statementsetting forth the facts, conditions, and circumstancesrelating to the accident or incident as they appeared.If the crewmember is incapacitated, a statement mustbe submitted as soon as physically possible.

e. Where to File the Reports.

1. The operator of an aircraft must file with theNTSB Field Office nearest the accident or incidentany report required by this section.

2. The NTSB Field Offices are listed under U.S.Government in the telephone directories in thefollowing cities: Anchorage, AK; Atlanta, GA;Chicago, IL; Denver, CO; Fort Worth, TX;Los Angeles, CA; Miami, FL; Parsippany, NJ;Seattle, WA.

7−6−3. Near Midair Collision Reporting

a. Purpose and Data Uses. The primary purposeof the Near Midair Collision (NMAC) ReportingProgram is to provide information for use inenhancing the safety and efficiency of the NationalAirspace System. Data obtained from NMAC reportsare used by the FAA to improve the quality of FAAservices to users and to develop programs, policies,and procedures aimed at the reduction of NMACoccurrences. All NMAC reports are thoroughlyinvestigated by Flight Standards Facilities incoordination with Air Traffic Facilities. Data fromthese investigations are transmitted to FAA Head-quarters in Washington, DC, where they are compiledand analyzed, and where safety programs andrecommendations are developed.

b. Definition. A near midair collision is definedas an incident associated with the operation of anaircraft in which a possibility of collision occurs as aresult of proximity of less than 500 feet to anotheraircraft, or a report is received from a pilot or a flightcrew member stating that a collision hazard existedbetween two or more aircraft.

c. Reporting Responsibility. It is the responsi-bility of the pilot and/or flight crew to determinewhether a near midair collision did actually occurand, if so, to initiate a NMAC report. Be specific, as

AIM10/12/17

7−6−3Safety, Accident, and Hazard Reports

ATC will not interpret a casual remark to mean thata NMAC is being reported. The pilot should state “Iwish to report a near midair collision.”

d. Where to File Reports. Pilots and/or flightcrew members involved in NMAC occurrences areurged to report each incident immediately:

1. By radio or telephone to the nearest FAA ATCfacility or FSS.

2. In writing, in lieu of the above, to the nearestFlight Standards District Office (FSDO).

e. Items to be Reported.

1. Date and time (UTC) of incident.

2. Location of incident and altitude.

3. Identification and type of reporting aircraft,aircrew destination, name and home base of pilot.

4. Identification and type of other aircraft,aircrew destination, name and home base of pilot.

5. Type of flight plans; station altimeter settingused.

6. Detailed weather conditions at altitude orflight level.

7. Approximate courses of both aircraft:indicate if one or both aircraft were climbing ordescending.

8. Reported separation in distance at firstsighting, proximity at closest point horizontally andvertically, and length of time in sight prior to evasiveaction.

9. Degree of evasive action taken, if any (fromboth aircraft, if possible).

10. Injuries, if any.

f. Investigation. The FSDO in whose area theincident occurred is responsible for the investigationand reporting of NMACs.

g. Existing radar, communication, and weatherdata will be examined in the conduct of theinvestigation. When possible, all cockpit crewmembers will be interviewed regarding factorsinvolving the NMAC incident. Air traffic controllerswill be interviewed in cases where one or more of the

involved aircraft was provided ATC service. Bothflight and ATC procedures will be evaluated. Whenthe investigation reveals a violation of an FAAregulation, enforcement action will be pursued.

7−6−4. Unidentified Flying Object (UFO)Reports

a. Persons wanting to report UFO/unexplainedphenomena activity should contact a UFO/unex-plained phenomena reporting data collection center,such as the National UFO Reporting Center, etc.

b. If concern is expressed that life or propertymight be endangered, report the activity to the locallaw enforcement department.

7−6−5. Safety Alerts For Operators (SAFO)and Information For Operators (InFO)

a. SAFOs contain important safety informationthat is often time-critical. A SAFO may containinformation and/or recommended (non-regulatory)action to be taken by the respective operators orparties identified in the SAFO. The audience forSAFOs varies with each subject and may include: Aircarrier certificate holders, air operator certificateholders, general aviation operators, directors ofsafety, directors of operations, directors of mainten-ance, fractional ownership program managers,training center managers, accountable managers atrepair stations, and other parties as applicable.

b. InFOs are similar to SAFOs, but containvaluable information for operators that should helpthem meet administrative requirements or certainregulatory requirements with relatively low urgencyor impact in safety.

c. The SAFO and InFO system provides a meansto rapidly distribute this information to operators andcan be found at the following website:http://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo andhttp://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/infoor search keyword FAA SAFO or FAA INFO. Freeelectronic subscription is available on the “ALLSAFOs” or “ALL InFOs” page of the website.

chapter 7. safety of flight - amazon s3 · meteorology 7−1−1 chapter 7. safety of flight...

Documents